Hebiki and OVA- do you really mean short span arms or are you actually talking about mid span?
Talking about using them for faster transition is making me want to try some now, I often feel my cars a little slow in slow chicanes and s-bends. Another thing to add to the expensive shopping list
I run my car outdoors in the summer and haven't used the diff covers in a long time. We run a lot on unprepared carparks, the most they get is a sweep for the dirtiest bits. I've not had any major problems but I have had a few stones stuck in the teeth of gears. It usualy means the other gear gets a bit taken out of some other teeth where the stone has been but the gears are still usable. I've actualy had stones get stuck in teeth when using the diff covers which makes flicking them out a real pain, without the covers you can quickly see where the problem is and fix it with a small flat head screw driver in seconds.
You can see an example of where I race on www.arccc.org
where a couple of videos have just been put up. The 15MB one is the 19Spec A final start with me in it
I'm the black and flouro red car with some metalic blue, starting third on the grid. There's also the modified A final but thats a huge file since it's the whole race, and doesn't have the close racing of the 19's