So this is an interesting one...how fast you CAN spin a HC is not the same as asking how fast you SHOULD spin a HC motor.
Strictly mechanically speaking, the valvetrain is not gonna like operation above 6750 or so. That's a LOT of RPM on a big 2-valve motor if you think about it.
On the other hand, without supporting mods and such there's not much to gain up higher either. So why go there?
Adding boost ONLY should move the HP curve straight up, not up or down the RPM range. However, it's virtually impossible to do that as you need more fuel with the boost unless you like running way lean and own stock in an aluminum piston foundry.
Not sure if that totally answers your question, but I hope it helps. Post back up as needed, and welcome to the forum!