Warning: If the Turbo suspension feels perfect to you, stop reading now. My "dissertation" is subjective in nature and might be upsetting. For me, one troubling thing about the Turbo is that besides the tremendous power and torque, over the long run, it's not a fun car to drive. The handling is a tad sloppy, IMO a result of the soft spring rates and the enigmatic PASM system. The steering is good but feels inferior to my 997 C2 in feel and feedback. My suspicion was confirmed after a test drive in the GT2: Porsche has elected to make the Turbo too much of a luxurious grand touring car. My baby feels like a Mercedes after a 35 mile drive in that incredible GT2; no kidding.

What I've done to my car so far:
1. Bilstein PSS10 with a height drop of 20mm.
2. Street GT3 alignment, meaning a change of front camber from -.4 to -1.2.
3. GMG anti-sway bar; set to full soft front, full stiff rear.
These are common and fairly conservative changes so I am not blazing a trail here. You are basically buying Bilstein's vision of what a mostly street-use Turbo should feel like. Consider what they do for a living, this is not a bad place to start. Technically, with its mono-tube dampener and dual spring setup, the Bilstein PSS10 is an advanced design.

While the effect of the sway bar is subtle, the PSS10 brings significant and easily noticeable changes. Some of these changes were expected-the Bilstein is no doubt geared more towards performance and its spring rates AFAIK are close to those of the GT2/GT3 class Porsche, others came as a surprise.

Handling:
While the stock Turbo could take simple corners with tremendous speed (as long as slow-in-fast-out is observed), when faced with combination curves, for example a fast S-shaped left-right combination, it staggers like a drunkard.
That the rear of the car likes to "walk" laterally while cornering is not that big of a surprise; after all, the engine is hanging out back there. The problem I have is that there are considerable vertical motions as well. In fast & bumpy corners, the rear has a mind of its own, and that mind belongs to a mambo dancer.
The Bilstein PSS10 and minor alignment change have completely transformed the Turbo's behavior. The suspension feels taut. Rear end motion is now 1000% better. In corners, problem with the stock car's tendency to understeer has markedly improved. Please trust me on this (grin)--I used to be skeptical too, the turn-in in corners feels different and better. Above all, it is now fun to take twisty roads with this car.
There is no more excessive front to back weight transfer with acceleration (squatting), and no more excessive side to side leaning in corners. It feels WONDERFUL.


Steering:
Before the suspension change, if I roll the steering wheel gently back and forth at say 70 mph, there would be some undesirable free play, or lag, to the car's response. After the steering wheel is turned, the car would take a split nanosecond before it changes direction, then it continues to lean a little bit after I've stopped moving the steering wheel. In addition, the steering is *too* comfortable in a curve: soft, even flabby maybe, with less feedback than I would like.
I do not know which of the 2 mods, alignment change versus Bilstein PSS10, is responsible, as unfortunately my planning didn't pan out and both those changes were done at the same time, but the steering response is now sharp and much more instantaneous. The feel is tight, and feedback during turns is outstanding.

Subjective/Objective Power Increase
The modded Turbo for whatever reason feels quicker & I don't have the need to always turn on Sport Mode like I used too. My speculation for this observation, 2 reasons: Since the car doesn't squat as much when throttle is opened, one, traction of front wheels is maintained for more power delivery, and, two, perhaps more power of that first instance of acceleration is going to the wheel, instead of being wasted in lifting the front of the chassis. Or it could be entirely in my head.
Whether this observation is subjective or objective in nature, bottom line is the effect is easily noticeable. Plainly and simply the car's acceleration when you step on the gas "arrives" quicker, with less lag. This then is the biggest and most pleasant surprise of this suspension mod.
Since there is less diving under braking, I would imagine that braking is also more efficient since the rear wheels are now stay put instead of lifting, but I do not feel this at my amateur level of pre-corner braking.

Bump Management & Noise:
Bilstein can't perform a miracle; you will sense more road bumps. However, the feel is vastly superior to the jittery ride of PASM-Firm and is entirely acceptable for my daily 35 miles drive to or from work in Los Angeles. Road bumps create a more compliant feel & conventional thump sound (yes I am the type of person who listens to my car's suspension--LOL), versus PASM-Firm's choppy and somehow unnatural feel, and the louder noise when the car hits things like freeway's expansion joints. For comfort level, if PASM-Normal is 10 out of 10, then Bilstein would be a 6 to 7, and PASM-Firm 1, tied with a Sherman M4.
There is also more road noise transmitted to the cabin, and for whatever reason, the exhaust noise sounds louder. Perhaps the smaller space underneath the car (from the lower ride height) creates some boom box effect.

With the above suspension mods, IMHO the Turbo could now more seriously compete with the GT2, if not in tracking prowess (AWD & Turbo weighs too much), then in FUN FACTOR. The GT2 is an all-out track car with a "heavy machinery" feel to it-heavy clutch, heavy gear shift, heavy (but fantastic) steering feel. The modded Turbo has advantages & disadvantages inherent to its AWD system, and despite of its avoir-du-pois problem, feels surprisingly agile & light footed. For my amateur skill and speed level, cornering behavior is now similar between the two.
My Turbo baby used to feel ponderous and soft, now it feels tighter, cat-quick, and lots and lots and lots of fun. Steering and suspension feedbacks are significantly improved over stock, and of course, that nuclear power plant that disguises as the engine is as stupendous as ever. I am grinning from ear to ear every time I come close to this car. The grin-score is so much higher that I would give the Bilstein an unqualified recommendation and rank its usefulness higher than PCCB brake, power mod, exhaust system change, and after-market wheels. To answer the question of the thread: Yes, for me the Bilstein PSS10 is a must if attributes that make a sport car feel sporty are important to you.