I shift my tko600 over 6000rpms, never had an issue. Know plenty of others that do it too.
There is alot of rumors about it shifting poorly above that rpm, but few people that actually use a 600 that complain of it.
Now i don't know my real shift points and i drive more by feel and sound than by visually looking, mainly because if i'm at 6000rpm i'm probably on the car pretty hard and don't want to land up in the bushes.
Also i use a tach adapter to run the stock tach, it's accurate at low rpms, but who knows up top. But i do know my power holds past 6000rpm so...
Taken from Tremec literature:
Do not attempt to shift the transmission when the engine is revving over 6,500 RPM’s.
I didn't even add the underline. The TKO was never meant to be a high RPM transmission, at least in stock form.