::026:: to what Snakebitten said:
Transforms the feel of the bike, talk to Simon at Race Engineering as he has flashed several S10's, it makes a huge improvement to low down in the first 3 gears and massively improves mid-range throttle response in all gears, I am convinced it is also smoother - and it definitely uses 6-7% less fuel.
Rear shock is also well worth the money for two-up, but solo OE is OK - but try it with two extra lines of pre-load wound on for tarmac use - you should find the handling feels much sharper like that, that is about perfect for someone in the 70-90kg range, if you much heavier you may want to add a shade more, two up you will find even max'd out it is soft and wallowy.
I also had forks re-worked by MCT in stowmarket, this gives a far more plush feel, with less sudden dive and better bump handling, but this is not essential, but if you have the cash and plan to keep the bike for years to come head south and get Flash and suspension done at the same time as Race Engineering and MCT are fairly close together.