Most people with the Williams have done little beyond the filter/exhaust. Thats due in part to the fact that the 16v/Williams are expensive to tune, but also because the Williams is already a cult icon.
If you want to mod a car then the Williams is not the best choice. Many people will agree when I suggest that modified Williams are deservedly rare. The Williams was a special edition of the Clio 16v, which while it was damn fast lacked low-down torque. So Renault made (in the end) 1200 Williams that didnt have much more power, but did have a much better torque spread. They basically took the best hot hatch of the time and made it better.
In a straight line and on looks, the Williams and 16v are hard to separate (both 0-60 in 7.7secs). But in terms of flexibility, the Williams wins every time. The Willy also has different front suspension and gearbox (many parts from the Clio/19 diesels!).
Its a bit more socially acceptable (on here at least) to tune a 16v, though very few do the outside as the huge arches, bonnet bulge and bumpers are already enough fore most. Tuning is still very expensive. Induction kits usually lose you power/torque as the cars run so hot. The next step are cams/head/chips etc, though a decent leap in power will cost you at least £1500 - and thats before brakes, transmission etc.
