i know that one of the guys that races a 998s and manages some wins, has had the heads modified, if i recall correcetly its as you said swish & dailed cams, (envolves a small amount of machine work) & i believe they use diferrant valve collets also.
certainly makes some differance , but not sure what you are expecting when you refer to a big differance.
i did enquire about having my 996r done but never got round to it, i cant recal how much it was though, someone else might help you more here
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