Interesting. I run the exact same set up on my Challenger, and the braking is markedly better than with the small disks. It doesn't feel much different under normal braking conditions because of the power brakes, but it stops much faster under harder braking conditions. Some of that is the larger tires though, if you were locking up the small brakes then there's no reason why you wouldn't do the exact same thing with the larger ones, and you might not notice a difference. With 275/40/17's grabbing the pavement all the way around, the big brakes on my Challenger really get a work out.
What pads and shoes are you running? If its new stuff, is it semi-metallic? A change from semi metallics on the small brakes to organic pads on the large brakes would definitely negate some of the advantage. Nice new brake fluid? Rebuilt calipers? New wheel cylinders? Rear brakes adjusted properly? New brake lines? What size wheel cylinder bores?
Leaky seals, contaminated fluid or pads/shoes, old rubber brake lines, air in the lines, leaky vacuum booster if power, all will do a number on your braking.