"Maybe a better way to ask...Is there anything I can hunt with #55 that I couldn't just as easily kill with #51?"
Well Brad, this is a very open question. I know hunters who have taken bison and moose with a 46# bow. I also know professional guides, who would not take a hunter after bison with less than a 75 pound bow.
If you shoot a big animal with a lighter bow and dont hit any bone or tendon, hair clumps, tightly flexed muscles, etc....an arrow can zip right through.
But that does not always happen. You might be the best shot in the world, but after your perfect release, the animal does a weird move and you just got a "less than perfect" shot.
This is why good hunters shoot as heavy as they can. Anything can happen at any time while hunting.
If you can build yourself up to shoot 55# accurately, you should. If you just cant, shoot that magic weight that gives you the best accuracy at the most weight. Be it 100 lbs or 40 pounds.( As long as its legal!)