wheel dimensions

Disclaimer: I am just starting to really get into my build, I've got some minor upgrades (suspension), but mostly cosmetic & general maintenance under my belt. Currently have stock 15in wheel on 31in A/T BFGs on a 2.5in suspension lift. Tread is running low (along with some other issues like...

I currently have P235/70R15 on my TJ and so far they have been great for highway driving and mild trails. I'm thinking about getting 31 inch tires once I save up enough cash and time. Will there be a big difference on the highway going from 28 inch to 31 inch tires?

I have stock moabs 16x8 and they have 5" BS. The wheels I was wanting were 17x9 with 4.5" BS. Comparing similar sized tires, how much would the 2nd example move the tire/wheel outwards? I was thinking 1" (.5" less BS, + half of the wheel's increased diameter) further out. So 1" but a DT rep...