Hi Tony. I've never run the xz speeds, stock or modded. Had many vzb speeds though. Neal has a new p/s/r in his shop for my C6 USA that's getting his newest version mod. Now that's one heluva runner. The last p/s/r he did for me i asked for it to be conservative and it ran very nice. Prolly more of a buggy mod as it was tamer than my Fire 11 i ran this yr. His newest mod however is much more aggressive since i only run truggy now. Too bad the C6's aren't around anymore. The new engines have the crank way over timed and don't respond to modding as well as the C & B series do. If you have a new p/s for your USA call Neal and bug him to mod it. It's a beautiful running engine. It'll probably be 2 more yrs before he gets mine done(lol Neal!)but i'll let you know how it is. The clocked F11 is a sweet ass runner.