Recoil of the .45acp in a Tommygun is pretty light, because the gun is so heavy. The "lightweight" guns go a whole pound lighter than the steel ones, to me, that's not saving much. It is a big difference when you are talking 6lbs vs 7lbs, but not nearly so much at 11 vs 12lbs!
I would think the semi auto SBR lightweight would not shear off a well mounted Eotech from recoil. Although how you mount it (and where) will make a difference. But if something wacks the sight, (like being dropped), it might tear loose, and then you got real problems. Likewise, you can even strip the threads in the aluminium receiver just by over tightening the screws.
The stresses on the receiver from a semi auto are different than those from a full auto, I have never heard of a semi failing due to the Al receiver, but I suppose its possible. Likely we don't hear about it much, because the guns don't get shot all that much.
I assume you would be wanting to mount the Eotech in place of the regular rear sight (don't see where else you could put it
). To a purist, it would destroy the classic lines of the gun, but so what? I put eversize rubber grips on my single actions (to improve shootability) and scopes on some lever guns, for the same reason. Its your gun, make it what you want it to be. I wouldn't do it to a collector's piece, but a current production commercial gun? Why not?