if a cooler is very good at cooling the tranny fluid then it could potentially cool it too much, which is also very bad for a transmission believe it or not, a tranny works best & is at it's strongest at operating temperature, too cold or 2 hot is no good, so some good coolers have a thermostat that won't open until the fluid reaches operating temperature. I ment that is the best kind of cooler to get.
& yes, a high stall converter is the torque converter. the reason your changing it is to allow the engine to reach higher rpms before the converter really engages, it's kind of like when you let the clutch out. stock converter is like starting to let the clutch out at 1,200 rpm's/ fully engaged maybe 2,200 rpm's, while a 10" converter may be like starting to let the clutch out at 1,800 rpm's, & fully engaged at 3,400 rpm's... obviously the looser converter is going to help with acceleration but it also adds a whole lot of heat... every converter is different & some are not much looser than stock really, but the higher the stall speed the higher the heat generated with even normal driving. upgrading the TC should def get a cooler