There are quite a few Barra's spinning to 7500+ rpm on hydraulic lifters. As has been mentioned, cams need changing if you want to make power past 6000rpm. The Atomic stage 5 cam will make power past 7500 rpm from what I've seen. On solids, you should make a little more power and gain some rpm as well. Are they "needed"? Depends on what you are trying to achieve. It's been proven that 600+rwkw is achievable on stock heads, cams and hydraulics with a sub 80mm turbo. As I see it, things like cams, solid lifters, porting, extra machine work etc will give you efficiency and therefore make more power with the same/less boost giving you a greater power potential before things like heads lifting and stuff breaking happens. That's why we're going to the lengths we are going to.