I've done some back to back tests here so can help. Running sintered over bonded made my speedo and motor run cooler but by different amounts. The motor ran 10deg F cooler, which is nice and helpful. The speedo ran just over 20deg F cooler which is really quite significant. Went from warm to really barely warm at all.
Now, you may be able to get away without the fan in 2wd but it’s a risk. There are several options; firstly in the B4 you can easily run a 25mm fan in front of the speedo taped to the chassis blowing through the speedo. You can I believe just wire it to the fan output on the speedo because it just needs that extra bit of cooling over no fan that you need for safety. I think this will be fine in 2wd however if there is still a problem you can wire it to 7.2V which makes it shift normally about twice as much air and will defo sort the issue. The other way is what Bradders did and milled down the centre fins, you can then mount an 8mm thick 25mm fan down the centre and it will just fit under a B4 shell.
In my X5 I can’t fit the 30mm fan either, I have a 25mm fan wired 7.2V pointing directly at the speedo fins (speedo on side) and on the weekend with a bonded rotor it was coming off less than 100deg F, i.e. VERY cool indeed, this was running a 5.5 motor and the car was very quick.
I do actually have a 4.5 but with decent cells outdoors I think it will get used few and far between. Will be fun though when I do
