I've heard of guys using Greenbacks with their 5150s without a problem. I seem to remember My Dying Bride using 5150s with Greenback-loaded Marshall cabs, but I could be entirely wrong. I know Alice In Chains guitarist Jerry Cantrell has used Greenbacks forever with all manner of stuff that probably puts out more than 100 actual watts when turned up. 5150, SLO, Dual Recto, Mesa 2:90, etc...
I know an old non-master volume Marshall Superlead cranked up all the way is throwing down significantly more than 100 watts.