I can't seem to find the data, but back when round balls were the thing I remember how little energy there was in a 45 cal round ball compared ot a 50 cal....... I mean it is a big difference and IMO a 45 round ball has marginal energy for deer. I know someone will disagree........ Thankfully, this is America, so we are free to disagree.
I don't think it makes nearly as much difference now that we can hunt with conicals and soboted rounds. I have both 50 and 54, and hunt with the same bullet out of each - just different sabots.