I've managed to get mine to within a second and a half of the fastest guys running 17.5 blinky at the Cotswolds track.
I have a few other things left to try, and my chassis is currently 150 ish grams overweight.
The mod that made the biggest difference is the lightweight suspension set (#53794). It's obviously lighter, but the front and rear arms are much stiffer.
I have an SSG chassis to try and I also bought some super shorties to get to the minimum weight.
For a front spool, you could try getting hold of a gear diff & gluing it solid! I wouldn't try that with a ball diff, as they're rarer than rocking horse poo
