Couldn't find any load data on the reloading pages utilizing bullets heavier than 500 grains. Anyone here fool around with a .450 Ackley using 550 or 600 grain bullets? I assume there is enough case capacity to push the heavier bullet at a meaningful velocity (2100 fps +). Any information will be appreciated.

I don't have one, but in Wildcat Cartridges vol.2 there is some loadings.
This is for 600 grn
79/N-203 - 2049
83/IMR-4320 - 2128
90/N-205 - 1979
85/W W-748 - 2107

I ran a series of loads with H4895 and Woodleigh 550 softpoints in a 22" Montana 1999. All with A2 brass, Rem 9 1/2M primers, 3.640" oal.

75gr - 2003.4
78gr - 2074.4
80gr - 2139.8 (2171.8 in 23" brl, long story involving Butch's Bore Shine)

Also, tried one load of RL15, 76gr - 1967.5fps.

The 80 grain charge seems a little hot so I settled on 78.