cam, intercooler, heads.
The M90 sucks that much, not the cam.
There are apparently some magic tricks that can be performed to get past 350, like race gas. But people don't like to disclose those tricks.
Have you ever done either?
Heads are pretty easy and straight forward compared to a cam.
Unless you pull the engine.
300 chp is easy.
I made more than that on a gen III and L67 TB.