well, the basic answer is accuracy and repeatability of the delivered pulse of fuel over varying voltage and temperature conditions.
the smaller injectors take less force to open, so the stock style hi impedance magnetic coils have more than enough power to pop them open and shut repeatably, and the loss of magnetic force at higher temperatures has less of an effect on the delivered pulse of fuel. Also, the larger the injector, the more fueling error you get from an inconsistent pulse of fuel. On a race car, idle consistency, and cold starting stuff is less important, but in a street car they are more annoying.
The low impedance injectors with peak and hold drivers are very in sensitive to voltage and temperature differences, and the coils have more magnetic force so control the actuation of the mechanism much better. The switching to hold current reduces the heating of the injector as well as lets it close faster (the magnetism needs to decay before the injector will close).
in the old days, anything larger than 40 lbs or so was unavailable in high impedance, nowadays there are lots of injectors in all kinds of sizes. Much above 80lbs doesn't seem to work very consistently, and even the 80's are quite temperature sensitive. There are lots of people using them with good success though(I'm probably too picky).
For anything above 80, I think you really have to run low impedance. I haven't seen anything larger in high impedance that works reasonably. 80's seem to be the current breakpoint. 80's are probably the minimum size for E85 for a fairly fast street car.
I will likely need to step up from my 83's this year, since the new engine is going to be a bit stronger than the old one.
Bob