I don't think the YFZ rear caliper fits. Properly working Banshee calipers are actually pretty good. It's just that the YFZ fronts are better. If you want, you can put the right front Banshee caliper on the rear. It doesn't work any different. It just looks a little cooler. You have to use the rear brake mounting plate. If you need a rear brake mounting plate, you can get it new from East Coast ATV.
http://www.ebay.com/sch/i.html?_nkw=banshee+rear+caliper+bracket&_in_kw=1&_ex_kw=&_sacat=0&_okw=&_oexkw=&_adv=1&_udlo=&_udhi=&_ftrt=901&_ftrv=1&_sabdlo=&_sabdhi=&_samilow=&_samihi=&_sadis=200&_fpos=&_fsct=&LH_SALE_CURRENCY=0&_sop=1&_dmd=1&_ipg=100
The 1 down side is that the bleeder screw doesn't end up at the top, so to bleed the brakes, you have to take the caliper off, stick something between the pads and bleed with the bleeder at the top. As far as your aftermarket rear caliper, post a pic. Some are junk.