I personally swear by Redline drivetrain lubricants, for everything from mild street use to ~500bhp S/C'd track use. It has never failed me, and it is absolutely worth the cost.
I use their D4 ATF in my 328Ci's 6spd Manual (swapped from 06 330Ci ZHP) and used it in the factory 5spd manual before that. In the original trans, there was a noticeable drop in temperature as measured by an infrared heat thermometer (6 locations compared), ranging from about 5-20*F. These were readings taken from the housing, not the fluid itself, but both were done on 80*f days immediately after a 45min (28mi) drive that was exactly the same route both times (2mi backroads, 10mi highway, 8mi backroads, 2mi hwy, 6mi backroads; this is the same exact route I use whenever I am testing something new out as the hwy gets engine oil temps to Op Temp, and all of the backroads are extremely twisty, like insanely so, and this combined with no one else ever being on them amounts to a good testing route...).
That had me very impressed, especially considering that it is similar to what an average trans cooler will give.
I also used their 75w90 Gear Oil in the differential up until I had an LSD made and installed (now I only use the fluid I get from Quaife as doing so warranties a rather pricey hunk of metal!). Same thing happened, I noticed a drop of 5-10*F via infrared thermometer. After installing a Rogue Engineering Aliminum Finned Differential Cover, the total temp drop was 22-28*F, very impressive IMO.
In my 325i, which has its original 5spd manual and non-LS diff, these fluids have been very beneficial. The car is my daily driver (150,135mi), and I keep meticulous records of it as with all of my cars.
When I switched the driveline fluids to Redline ATF and Gear Oil, with the only other change being a fresh batch of RP 5w30 + K&N Oil Filter ("old" oil had 3892mi on it), my average fuel economy went from 27.385mpg to 28.675mpg as measured via both my own calculation and the onboard computer (I always use the same pump at the same Shell as it has 93oct pure gas, no EToH). Weather was similar, and there were no other differences... Fluid that was replaced was OEM BMW fluid from dealership that had 38,228mi on it when dumped.
Other benefits have been: smoother shifting (both w stock shifter in 325 and UUC Evo3 Short Shift Kit + UUC Dual Sheer Selecting Rod + UUC "Race" Tranny Mounts), better economy, less difference in shifting due to temp, lower trans and diff operating temps, and Peace of Mind!
Everyone has different views, and its possible I could have had similar results with a different brand of fluid. I am just providing anecdotes, so don't assume you will have the same experience as me, although almost all of my friends now use RL fluids (in everything from Mini Cooper S, 350Z, S2000, Audi A4 1.8T, Audi S6, BMW 330i, Mercedes C32 AMG, and more) and all have found it to be excellent.