Think SIMPLE. Porsche has to earn money. There is no need whatsoever to put the GT3 engine into a Boxster or the Cayman. What for? There is no official Boxster or Cayman Cup...yet.
With the GT3/Turbo/GT2 things are different, even different customers. They know what they buy and they usually care, especially the GT3 customers. It would be pretty difficult to sell them a modified 997 Carrera engine instead of the "real thing".
How a future Cayman CS may look? Well, maybe same weight (I doubt weight will be lower due to safety regulations worldwide), at least 40 horses more and a modified suspension and MAYBE a limited slip differential.
I don't expect more but let me ask again: what for? Porsche doesn't seem to have plans to race the Cayman, at least not now. And for a couple of customers who want a light-weight stripped off fast little racer, it is not worth it.
We should be realistic: there won't be a car priced below the Boxster. Period. So if you're hoping for an affordable Cayman club sport, even as an "S" only, forget about it.
There will be an entry level with rumored 250 HP and thats it. Period. Of course this car costs a tad more than the Boxster.
Again: Porsche is in the money earning business, they don't have anything to give away for free. And there are a lot of people out there who are willing to pay the prices Porsche asks for their products. I don't always agree with Porsche and I think that the current 997/987 model line is not what customers want (even if they buy it...), especially regarding power figures. But what the heck...I'm just another customer, who gives a damn about my opinion.