I have no experience with bumpers. (Hanging on to my stock chrome bling!!) However I did use a billet grill for a while, and it flows measurably (in temperature) less air than the stock one. So I cut out what I had on the stock one to use it with my transmission cooler fans.
Those fans are still crucial for me while crawling up steep trails, but they do work well. They are not needed above about 25mph. I have not tried manually actuating the main fan clutch, but if one of my fans dies, I'll investigate that.
Other, perhaps useful info:
There are (at least) two ways to measure temp on the 4r100, plug in the pan and internal sensor on the exit. A scan-gauge or similar will read the internal one, which is the hottest
point.
It's not uncommon to see temps over 200 there, but I don't think they are great, certainly not for long periods. I wouldn't stress topping out at 200 or 210 for a moment at the end of a big climb. At roughly 230 the pan seal will start seeping, I don't know what happens if you don't let it cool down at that point.
Keep in mind that all of that experience is with my _previous_ transmission. The new one seems to not get as hot and cools much better, probably because it's not clogging the coolers or slipping.
I have yet to do any towing with my new transmission, but without, I'm at 130-160 when previously I was at 180-190. However, given a long enough hill and a heavy foot, temps can still be driven up.
I don't believe too low a temperature is an issue, unless you are under 120, and the transmission won't shift into overdrive, or lock up the converter. If so, put a bit of lexan over part of the cooler during the cold months.
I'm also on a v10, 4r100 (bts now) and the big tru-cool after the oil to water cooler.