Anyone ever run this Purple shaft?

If it completely noses over at 5500, it could be the valve springs are not quite up to snuff. Honestly though, I've put together several engines like yours, and they do tend to top out in the 5500/5700 range. I'd stay away from the cam your talking about at this point, unless you want to do a lot of racing. I understand going to the top of 3rd or 4th gear with your combo.....that's some serious mph :D.

Bottom line, the cam you have is a decent off the shelf grind, and some timing and carb tuning may help with how it feels.