It's funny you should say up there because it was Simon @ Sitech Racing that advised me not to use them because they warp, this was a few years ago now though so maybe his opinion/experience has changed?

I'm guessing heat but as I mentioned in your other thread I've not actually tried one myself, I always use black top heads as these do over 400bhp and is more than enough for most people.

How do they drive with fixed cam timing, I imagine the turbo makes up for a lot of what's otherwise suboptimal VE?