22LR, HV @ about 1250fps: many shooters have a hard time getting as good accuracy from their cast bullet reloads as these, and none have GCs.
My CF cast reloads, WWt and range scrap with a dash of tin, PB or GC at 1250 dont lead (given the fit is right as mentioned above; with small doses of fast shotgun powder or slower stuff.
My .30cal. 1450ish fps loads do have GC, Equiv. Ly#2 alloy unknown lube but it is blue, and Motor Mica dusted (I got these in bulk from a com. caster some years back): no leading, and I could go faster, but I dont want/need to, so I dont know their limit. Ive done the shotgun powders, through 2400 and IMR 4227.
IMHO, GCs will maybe make a marginal fit (too small) projectile OK, and wont make a larger one bad, UNLESS the GC is seated on uneven on the base, a very common thing, BTW. It adds another variable in the mix that can be devastating to accuracy if done poorly. If done correctly, straight and square, it 'should be' as good or better, but against a proper PB cast at 1250 I might be the biggest detriment to accuracy.