I've been using a gear diff up front for about 2 months now with 500k oil in 17.5 stock rubber TC (decent size carpet track). Tonight in practice I put a spool back in and made some setup adjustments and the car is easily 3-5 tenths faster than it could've been with the gear diff. The spool is just more aggressive and feels better on a smaller tight track (compared to large asphalt tracks) because it has more 'get up and go' and it is easier to get the car loose.
If I were racing on a much larger track, I would run a gear diff. No matter how "off" my setup was, the car was super easy to drive and very forgiving. It held a line well, also. On a large flowing track you don't necessarily need the instant-power feel of a spool since you carry momentum more.
Just my experiences!
I have not ran a gear diff on the rear yet, but I may try it soon.
Stop holding bitterness deep in your bum soul.