OK, one man's story. This may not apply to your situation, but maybe...
I ordered my 2013 VS with four-wheel discs.
The fronts have Varga calipers and have been pretty much trouble free. The rears were EMPI's and have, well, NOT been trouble free.
We'll skip the really loud 'moaning' noise they made (not brake squeal) which was finally solved by swapping out the supplied cheap, stamped brackets for the more expensive cast brackets. We'll also skip any discussion of how incessant axle seal leaks (on both sides) suddenly disappeared when the cast brackets were installed. And we won't raise the question of why a brake manufacturer would make available TWO different brackets for the same brakeset, ship the cheap brackets with the brakes, and charge $90 a pop for the, uh, 'upgraded' cast brackets.
Mentioning any of that stuff might get me riled up enough to launch into a rant against EMPI products and marketing techniques. And I'm trying to stay objective here.
So, what was it again? Oh yeah, brake pads that seem too thick to fit.
Note that I say I 'finally' solved the moaning noise with new, cast brackets. The first thing I tried to quiet things down was different brake pads.
It turns out those brakes take the same size pads used on many cars, including a bunch of Fords (such as the 2005 Taurus, for example), so there are lots of after-market pads available that will technically fit. The size is usually referred to as 'D610' in the industry
I ordered a set of these Bendix ceramic pads from Amazon and they did drop right into the EMPI calipers. But, like you, I couldn't spread the calipers wide enough to squeeze the pads around the discs.
And, like, I tried.
A lot.
Not to be defeated, I ended up sanding down each pad about 1/8" until they finally fit. It's now about 25,000 miles later and they're still working OK. So, maybe the supplied EMPI pads are thinner than accepted industry standards?
That's another question I won't ask.