I do 8K on the CFM gear.
But bear in mind this isn't sustained RPM, just a very short burst.
And this high RPM most talk about, most of you aren't making any power up there anyways. So it's kinda irrelevant. Curious is fine and all, but you have to know that a stock engine/head isn't going to make power past 5.5K too well. You'll need a good head and cams to make power past that point and need the extra RPM.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.