My Flintlock with 42" barrel and 100 gr of 1.5 fg will. That's a longer barrel, but also a flinter with a gaping flash hole and MUCH coarser and slower powder.Never have heard of a patched round ball, pure lead, fragmenting due to excessive velocity. Not much in the way of expansion happening, either, though the ball will deform particularly if it hits bone. I can always be wrong (in fact, am practiced at it), but I don't think I am in this case. Using my 36" barrel, .50 caliber rifle, I hunted with 140grains. Almost without exception the ball would be found, deformed but not "expanded" under the hide on the far side of the critter.
At 100 grains in a GPR, there is about zero chance of getting to 2000fps. Barrel length is key - velocity will top out at the point all the powder burns just before the ball exits. More, and you just make fire out of the muzzle. But in the 32" bbl GPR, one should be able to work above 100 grains before just blowing powder out of the barrel. I don't think you can find any data, published or from a buddy at the range, that will show a .54 at 2000fps out of that GPR barrel. You are correct that 2000 is a hell of a lot for a .54, because it just ain't gonna happen in that rifle with 100 grains of powder.