I have shot smaller diameter shafts and can't see any benefit. Penetration is difficult to measure in the real world.. hard to simulate blood and tissue using gel or foam.
You likely will need to increase spine as it will move you closer to center.
You .. as most of us, probably won't be satisfied until you try for yourself. If you want to increase penetration, shoot them back and inch more.. or even two. When we crowd the shoulder, the vitals ...depending on the game animal, generally get smaller and are more surrounded by heavier bone and muscle. I'm very guilty of crowding the shoulder and rarely get complete pass through even though I shoot heavier equipment than most.
I mean, I always have two holes in them, but they generally twitch at the hit and break my shaft as it is coming out of their off side shoulder.
BigJim