Eh, so here's the question.... how fast do you NEED to shoot slugs? I got my MK1 .22 Crown up to 65fpe, and still could have gone higher. It was fun. But completely unnecessary. I put a shorter 500mm barrel on mine, and run about 50fpe with lighter slugs. (950fps) Easier on the gun, easier on my air use.
In summation, if I were to max out the capability of both guns (modifications included), the Impact would make more power. But not a whole lot more. The Crown is quite powerful compared to most pcps.