So my .300 upper arrived with a 1:8 twist instead of 1:7

[brutal]

I'd stick with the 1:8. I have a 1:7 that I can only shoot subsonics out of (its suppressed with the can buried under the handguard). It keyholes like crazy with supersonics, and I stopped after one mag when I very luckily didn't get a baffle strike.

I have a 1:8 that shoots 147s, 208s and 220s great.

YMMV

Good to know.

However, what length barrel?

My AAC branded 9" is 1:7, no issues, no keyholing subs or supers (that I recall.) However, I really only shoot subs out of it now, and mostly my own loads unless I let someone else run it, then factory REM subs.