You shouldn't forget that the Cayman S engine is different from the Boxster S engine, even if both are actually based on the same M96 engine base design.
But up to a certain point, you have a valid argument.
Of course a Cayman with 355 HP would be superior to the 997 Carrera S, especially if Porsche manages to install a limited slip differential and a slightly sportier suspension (not PASM) on the Cayman. But that's not the point. Porsche could build a lightweight CAYENNE at 1800 kg which could compete with the 997 Carrera S up to a certain level. The point is: WHO do you want to sell the Cayman to? WHICH is the target market? Young people? Porsche newcomers? Old people who want to feel young again? Former Porsche owners who don't like the 911? People who don't have enough money for the 911? How does the "typcial" Cayman S potential buyer look alike? Well: I could post a few sentences out of a marketing document regarding the Cayman S but it is confidential (as far as I know) and some people would feel offended again. The truth is: the Cayman S isn't supposed to be "better" than the 911. Period. But if there is demand, a sportier Cayman would be possible for the far future. Right now, all rumors indicate a Cayman base model for less money, somewhere between the Boxster and Boxster S.

I think this strategy isn't good for Porsche. The Cayman S should have had 320-340 HP from the start, the 997 should have had more than 355 HP from the beginning, no 325 HP base model. I think Porsche thinks too much marketing and earning money and not enough customer satisfaction.
No potential 997 customer would seriously consider buying a Cayman S because it has a mid engine and only two seats.
And some potential Cayman S customers don't want a rear engine design and two additional rear seats.

At the beginning, I thought that the Cayman S is some sort of "attack" upon the legend 911 to make Porsche independent, on a long term run, from their traditional sportscar. But with the Cayenne and the Panamera around the corner, I doubt that Porsche really has to worry about their future. So unfortunately for the Cayman, it is what nobody really wanted: a very nice and highly refined Boxster Coupe. Nobody really asked for it.