I'm not fully happy with the noise the spool makes. When running the diff, the assembly creates a little play which is crucial to the correct match between pinion and spur... with the spool and the spur gear screwed down there is no play left, and the coupling become noisy... how do you solve that?

This works well. The shoulder screws let the spur float the same way it does on the diff balls. Significantly reduces noise.

I'm running mod with xenon / axon gears as well, but the mesh is nowhere near efficient as the ball diff with same gears. Maybe those shoulder screws would solve that, giving that little play

Take a jewelers file, and make the holes your screws go through a touch bigger. Problem solved.

For 17.5 1/12th scale on medium to larger tracks is it true that no one is really running any brakes at all or using them in the corners? If so, are you setting any drag brake in the speed control? What setup changes are you making to the car to get it to rotate quickly through the corners?

In my 17.5 Destiny car the only time I use my push brake is if there is a tight °180 at the end of the straight or some similar corner. As for drag brake I have 4% on my hobbywing esc. I am now running a locked axle setup and have not changed any of the esc settings yet.
Now for setup it will depend on the chassis and option parts you are running. Best I can suggest is to find a setup sheet that corresponds to your track conditions and car that was made by "one of the fast guys". Or post what your current equipment and setup are and I'm sure you will get plenty of feedback here.

Edit: I also have the push brake turned WAY down on my radio. I don't have an actual number currently since I am at work.

I don't know what to say, my mesh is perfect with the spool. Do you have the gear sitting on a bearing anyway? Even though the bearing isn't turning it still centers the gear on the spool.

since i learned i was driving 12th wrong, it's been a while since i've braked during a race unless i know i just overshot a corner to hell

On black carpet with 17.5, I find that I barely flick off just prior to apexes and smash it back on for the first 45 seconds or so. Then it's full clamp for the next 7 minutes. I can not say that drag brake is even relevant anymore. Push brake is only in play if something needs to avoided/corrected. I can not say that I enjoy the current black carpet throttle strategy these days. Might just be me though.

13.5 has just a little more throttle control, but it's pretty much full smashed at 5 min. Mod uses the entire trigger.

Thanks for the input guys. I need to find out what I am doing wrong or where I have the car set up wrong. The last time I ran it I was using brakes in some of the corners, but still being new at it I very easily could have just been driving that horribly. I know your lines have to be very precise to be fast in 1/12th scale.