Basebands cannot be downgraded on an iPhone 3GS, as far as I know.
And as far as having 6.15.00 goes, I don't really see anything wrong with it. I own 4 iPhone 3GS devices right now with one running on 6.15 and used to own a 3G running on 6.15 and experienced no problems. I already have an awesome GPS anyway so losing that wasn't really an issue for me.