The most I’ve ever gotten out of a factory head is mid-270’s…….and that was an X head…….. which have a more sloping SSR than a J head.
I had some cnc ported J heads here that went 280-ish after some tweaking of the runner & VJ.
These heads had some fairly generously sized bowls.
Both of those heads had flow regression after the SSR gave up.
If I’m doing J heads myself, I’m done when they’re in the 260 range.