Whats your oil temperature situation on your N20? I did not see cooling modifications in your other thread
In the stock N54 on my car I hit limp mode and power reduction really easily, especially when there are lots of chances for full throttle, like "big willow"
I am speculating completely here, but can't help but wonder if better cooling and venting the engine bay will help out with this alleged head cracking issue. Or if the cracked part of the head was close to the downpipe