It is not a hp problem.
Just look your comparisons you are matching a z4M that is o the top of BMW lists under m6-m5-m3. For the performance issues. So you are talking about BMW's 3rd fastest car. But Cayman S is under the Carrera. Think about how many Carrera cars. Gt3,TT,Gt2,Cs,C4s,C4,C2.Also caymans performance is under the 996 Carrera's. So it is below the middle of Porsche list.
For the Z06 issue it is fast car yes but do feel so safe at higher speeds and on corners while driving a vette??
FOr me i will not.
My ex car was 996 C4S i can go up 280kmh(on digital speedo) very easily and i saw 301kmh(on digital speedo) and there is 200 rpm to go also. I was not afraid and feel very stable.
I agree for a 343hp car M3 is good. But their challenge to a porsche is not the performance. People choose bmw M's or Merc AMG because they are comfortable and 4 passengered, expect roadstars.
Porsche ignore roadstars for Boxster S they dont mind Z4M beats it. IMO they think if you want to have performance and convertible issue go and get Carrera Cab or Carrera S cab. It defenitely beat Z4 mCoupe/roadstar
So if porsche think that challenge is important they can build a Boxster Turbo or Cayman Turo about +360hp's it will defintley beat slk55 and z4m very easilly. But porsche think Carrera sales. Thats the point.

Porsche produced Cayman 2.7l. Because if they produce a car in the price area of z4 coupe and audi tt, people will get porsche brand. So thats the point also