Thanks for the replies, so far. The 2015 brakes do give me something to think about. I'd need to know more about the spacer dimensions to either make them or get them. Would I need different dust guards?
I, also, wonder if the 4 piston calipers would require a change in master cylinder. I don't think it would because in terms of brake design, 2 piston calipers (acting against a stationary pad) move the same amount of fluid as 4 opposing pistons, and, also, apply the same amount of clamping force.
But this option would increase the price of what I was thinking of doing (not nearly as much as a Brembo, or other big brake kit, but still). I am planning on upgrading pads and have to replace the rotors...(whether I get OE style or drilled is to be determined for aesthetic reasons really). What size rotors wouldn't really change the cost much, and the 2012 caliper brackets can be had for about $30 a set refurbed. I just have to look at a couple 2012 Gt's to see if the extra 3/4" looks any bigger. Bear in mind, I know the biggest improvement in stopping power is going to come from pads, so going to the 2012 rotors would mainly be to try to fill out the massive empty space in my rims. LOL.
The 2015 rotors, however, would give better braking feel even with the OE pads, so I have to see what more would be required to install them. I will be selling one of my motorcycles in the next month or two, so I will have a little extra cash burning a hole in a pocket......