It is an interesting problem, but there are too many undetermined variables. Your hand didn't ache after 600 rds of .45 in a day but it does after 750 rds of mixed .45 and 357SIG in two days. Probably a one day gap is not enough time to recover so we might think of it as shooting 25% more with 1 shot in every three a 357SIG.
Because 357SIG pistols are lighter than .45ACP pistols the felt recoil and muzzle flip is significantly more with the 357SIG than with the .45ACP even though the total momentum transfer is almost equal. Muzzle flip from the 357SIG might be twisting your support hand fingers in a way that is causing the ache, but it is hard to say how much this might be the case without seeing your grip style. Can you say how much more muzzle flip you see with the 357 relative to the 45? Can you name of describe your grip style?