No, certainly not stronger then any pistol.
No conventional lever-action is as strong as the strongest Magnum revolver designs, or single-shots, due to the way the bolt locks shut, and the amount of case exposed at the extractor.
I don't have any information on the Henry Big-Boy as to whether it will handle +P loads or not.
However, since they also chamber it in .44 Magnum, (36,000 CUP) I think we can assume it would be safe with +P .45 Colt. (25,000 CUP)
However, the best thing to do would be to call Henry and ask them.
rcmodel