Big Problem....
Gentlemen,
We are talking about several factors. I will list them:
1. The 300 degree limit is a certification issue. Due to the flash point of Skydrol (It varies a bit, based on individual manufacturer of the Skydrol.), AND due to the fact that the Bus hasn't wheel well fire warning, you are limited to 300 degrees of brake temperature for takeoff. (Typically, the flashpoint of Skydrol is around 425 +/-.) So, ostensibly, if you take off with brake temps at or below 300 degrees (fans-on or fans-off temperature), you'll have plenty of margin. (Yes, I know, you can't take off with fans on...you turn them off before takeoff...but, say you turn off the fans as you're taking the runway for takeoff...and the brake temp at brake release is 300 degrees...well, the actual temp of the brakes is around 425 or so.)
2. Which brings up the next point: Yes, due to the cooling of the brakes by the fans, you'll have an actual brake temperature of around 125 to 150 degrees more...than what is indicated...when the fans are on.
300+125=425.....What a coincidence!!!
3. It's true that carbon brakes wear better when they're a bit warm...not hot! Depending on the brake manufacturer (They all have slightly different engineering data.). However, the stopping capability of the brakes decreases with increases in temperature. Your best stopping capability is with brakes cold. You must not confuse stopping capability with wear...they're two different things.
4. The single-aisle Bus has a little-known caveat. The accelerate stop distances you see published are predicated on the brake temperatures (at brake release for takeoff) being no more than 150 degrees. So, you can take off with brakes at 300 degrees (and be OK with regard to fire in the wheel well), but if you're accelerate-stop limited on your takeoff, you might want to reconsider things. You may or may not get it stopped in the runway available.
If you follow the FCOM and FCTM, you can't go wrong. I hope this clarifies things.
Fly safe,
PantLoad