There is a ton of confusion on the stock wheels. If you measure from mounting surface to OUTER lip of the wheel, you get 6.25". But, if you measure from the mounting surface to the INNER lip of the wheel (where the tire bead would rest) you get 6.0". Also, the offset is stamped into the factory wheels. I think it says 44.45mm, which if you do the math translates to 6.0". So most people on the forums are measuring wrong.
Then that brings me to the aftermarket wheels...each manufacturer may measure theirs differently. So really it's a crap shoot. You never know what you're going to get.
I look at it this way...if the factory wheel is 7.5" and 6.0" of that is backspacing, then 1.5" is sticking outside under the flares.
A wheel that's 8.5" with 5.5" of backspacing (like the one you are talking about above) is going to leave 3.0" sticking outside under the flares.
My math above was totally jacked up and I apologize. But you are effectively pushing your tire out 1.5" PLUS the added width of the new tires, so they are definitely going to give you a wider stance.