Just built my new SIII....OH why have I been poring money into my SII to try and match this performance. With just running in the engine (very light on throttle) I managed to beet my average lap time by 1.5 seconds at my local track.
Compared to my SII with Nova cx12p the stock SIII is absolutely great...Easier to drive, great turn in on corners with a faster lap time. Looking at the difference the SIII puts the power down sooo much better than my SII, foams will last 100% longer. My SII was heavy (had lots of 3racing bits hanging off it). I now realise what a mess you can make of a car with stupid upgrades.
My SII had 4mm chassis, front blade sways, rear sway, front one-way, carbon top plate (battery & receiver under side of plate), v-one-r 3-racing rear suspension, v-one-R shocks, alum pulleys etc etc.....
Not sure about the SIII spool diff...(basically direct drive of front wheels, no diff). The only thing I would buy is anything that would lighten my ride significantly and maybe later a sway bar, however want to try and squeeze what I can out of the stock setup.
In hindsight I stuffed my SII by adding weight with the 3-racing stuff and shocks too stiff...Although it did look great
Sell your SII and buy the SIII