Wrong, even with a KA where it limits to ~5500 rpm with speed limiter if you jack it up and let her rip, the momentum from accelerating 0-5500rpm will proper the drivetrain way past the limiter, I have done this.
Also you wont make it much further than 8000 in 5th on the highway, given certain conditions you might, in 5th gear if you are running stock sidemount/turbo. You need a bigger turbo/higher cfm in order to stay in boost/make power to go that fast even if its just 7psi.
If you wanna check if its running lean, buy/borrow a wideband. There are countless reasons it could be running lean at times even if the timing is nice and idles fine.