Any difference in twist rate will be almost undetectable as far as pressures are concerned. Chamber dimensions and actual bore and groove measurement in .0001" increments will have a much greater effect. The leade or jump to rifling as well as the angle of the reamer's entrance to the rifling has quite an effect on pressures. Choice of brass being used (especially in .308) creates even more differences.
Many custom reamers have 1.0 to 1.5 degree cuts that ease the bullet's entrance to the rifling. A lot of factory chambers have a 3 degree or steeper cuts that abruptly jams the bullet in the rifling.
If you pick a half-dozen factory rifles and shoot identical ammo through each of them you will end up with well over 100 fps difference in velocities more times than not. They're same barrel dimensions, twists and reamers, just the end result is caused by small variations in barrel dimensions from one rifle to the next.
In theory a slower twist rate should produce lower pressures and lower velocities all other factors be identical. Getting all other factors identical is near impossible. I always buy faster twist barrels when I have a choice. It never hurts accuracy and it allows you to use heavier then normal bullets for caliber should you want to try them. I like 1/10 for .308 and 1/7 to 1/8 for 5.56mm. I own an 1/11 twist .308 that shoots great, but not better than my 1/10's. I would never buy a 1/9 5.56mm unless my only use for it was blasting away with combat style shooting.