Tire performance is the limit; the mgu-k can in theory brake the crankshaft as hard as the tires and power electronics can handle. Lighter is always better if everything else is equal, but that's not really the case with hybrids.
It also depends on what your point of comparison is. Current vehicles are able to go faster with a V6 than the old V10s could, and that compactness is saving weight and adding performance elsewhere, so it's not as straightforward as modern F1 cars being too heavy. Modern cars also have a much bigger focus on safety than the old cars did, which, again, adds mass.