Just ran some bare shafts through my bow and I'm getting nock right around 6-8 inches. I'm pulling about 43# at my draw weight at 25 1/2 inches. Arrows at uncut GT 3555s (500 spine) with 125 grain field points which is all I currently have. In your opinion would moving up to a 150 grain point be enough to bring the bare shafts center, or would I need more than 150 grains.

This was also done at 15 yds if that helps at all.

You will have to get up over 200 gr. for those shafts to work. I have a similar set-up and use .600 spine.

Yes 600s should give you better results.  I have one bow that I even use 1716s it is 43 lbs and wants a really light spined arrow.  You are doing the right thing paying attention to want your tuning tests tell you.  Check out some 1716s--- inexpensive and you should be in the ballpark.

I use uncut 3555 with 200gr points and broadheads out of my 42 - 46# bows and they are perfect for me. For lighter points, I use 600 spine full length (which are a bit shorter than the 500 spine full length).

Every bow is a little different, but I think yours is telling you it wants a weaker arrow. I'd buy/borrow some 175 gr and 200 gr points and see what that does for you before I bought different shafts. Points are cheap; shafts are not.