I agree that the Turbo will have the most depreciation, I think they are very aware of the maximum that they can price these models without going into Ferrari and McLaren territory.  It's the same way that most of the other manufacturers price their models as well.  For example, a loaded C-Class gets into E-Class territory, a loaded E-Class gets in S-Class territory, same as BMW's 3-5-7 series models.  

There are a couple tricks that Porsche has over the others, which are:

1) Daily Driver ability - You can daily drive a turbo(s) and put 12-15k on the odometer every year without it drastically affecting value.  If the turbo(s) is your 2nd or 3rd sports car than perhaps an exotic is more your speed.

2)  Less Flash factor - Porsche models range from $50k - $200+k, but you can get a used Porsche at a very affordable price, this makes the brand more available to the masses, and since the lay person can not tell the differences between model years or even some models, it allows you to drive more under the radar than any Ferrari or McLaren model from any year.  Further, if you work in a field that you do not want your employees or colleagues to think you are making too much money, it's hard to drive an exotic any more than on a weekend.  Moreover, you can park a Porsche in a parking garage or other area and be far less worried than if you parked your exotic.

These are things that Porsche knows and can use to command these prices.  I've been hearing that the car is all-new from the ground up and there's more expensive technology, we have yet to see if that's true or not.

Lastly, the depreciation for Porsche based on the last 997 turbo model is not unbelievable IMO, I ordered a 997 turbo back in late 2006 as an 07 model fully loaded for $155k, I sold it in late 2011 after driving it for 4 years and 8 months for almost $100k.  So $55k depreciation (not including tax, fees, and upgrades) isn't terrible to me for putting 37,xxx miles on it.