Personally i don't think one of these cars are worth owning without the rear operating correctly. Might as well be an old pickup truck.
And as far as not hurting anything running it the way it is, my buddy always complained in the past that if they were run too long non functioning (he did this for a living with foxes) that the pin could somehow get stuck and not want to come out, which may not be the end of the world at a shop, but could be hell in a driveway.
For arguments sake i texted him, the response i got was that if the spider gears got loose from a lack of tension is could be a issue. I know he really used to whine about back in the day, so i'm not sure the level of "issue" the problem is.
As for faster, of course it's going to be faster, you would be doubling the traction.
I also stand by what i said a page earlier (though i must be getting old, i didn't remember writing it).
The second you crack that cover, you should rebuild the entire rear. Do it once and get it over with, done correctly with a set of gears you will never do it again.