There is certainly nothing wrong with shooting something about 50# if that's what you can handle well and shoot accurately. You did mention good criteria though---"assuming good shot placement". I do agree with Elkslayer though. Lower poundage and mechanical heads are not a good combination.
I'm a bit anal about which broadheads I shoot and tune for and if you can decently tune your bow/arrow combination there is no earthly reason to shoot a mechanical head. Choose a good, sharp fixed or replaceable blade head, tune well and shoot with confidence. Some guys say two blade, but I have never had a problem with 3 blade heads and much prefer them in some of the narrower cut configurations---1 1/8" or so. With an arrow weighing around 350-400 grains you should have very few problems, IMO.
If You're Not Living on the Edge You're Taking Up Too Much Space
Martin/Rytera Staff Shooter
PSAA Life member, UBP Life member
PADI AOW Diver