Don't know what else might be at play here, but my old 331 definitely spooled up quicker than my 250. The 331 also had the forstrokes header, but I had the stock airfilter as well.

 

I spoke with Bill Blue last week and he offers head porting for an extra $400 (done by a machine shop near him). They port the exhaust ports so the 351 kit will spin all the way to redline. I'm probably going to have this done when I do my 351 kit hopefully in a few more months. I blew my mod savings on that FZ1 I got last month.

Rob