Hi mtiweg,
I must agree with you it provides better S12 and of course higher output impedance (if I'm not wrong) which is what I want.
Firstly, power gain has correlation with Fmax. with a cascode structure, a tunable inductor between the 2 transistors has been proven to increase Fmax by about 25% at 90nm tech node on a class A (sadly) PA. I am just bringing the idea across to try it on a class C because the biasing really made the power gain suffer by a lot. I got the gain expansion with a single transistor but it's just not enough.