Rocket Car man - If one wheelbase was magically 'fastest', then that would be the only wheelbase you would see Adult league cars using. The reality is there a LOT of wheelbases being used, with certain ones fairly popular, like 5", 5 3/8" and 5 3/4". I have not attended any races in person, so I am far from the expert, but I think that BSA standard short WB is pretty rare, unless the rules required it. I think there are other recent threads that address this topic.
Not sure why you are worried about COM. There is a sweet spot for everything and the only way you find it is with a track. COM should move back with increasing wheelbase, but many factors affect the "sweet spot".
Fenders would work with either of the wheelbases you asked about. They can certainly help, but you have to do them right (see other thread on this). I wouldn't waste sugar pine on them, my preference would be balsa but there are painting issues to overcome. Do you have access to a similar track ? I would certainly advise against putting your first set of fenders on a new car for an important race without getting some track time.
The typical 1st timer mistake (and most people included me did this) is to not have a 'relief cut' betweent he fender and body to make sure that there is no chance of the fenders contacting the center guide. See SpaceWalker's recent video which shows this. Again, I am not a fender expert (since I prefer cars with razor wheels), so this is just my 2 cents.
