As the lads have said if your cars dropping off power before this point then why would you want to rev it that high, your just prolonging the dead zone so to speak.

Sure it sounds good at 7000 RPM sounds better still at 8000rpm and beyond - but in my experience wont last long. I'll not go into failure details on here but we had a few with my builds. Your asking a lot with valves at the size they are likely to be and the weight they are and when you add to that the additional RPM its no surprise that there has to be a limiting factor somewhere.
Your beter looking at maximising your usable power band IMO