there wouldnt be too many bumps or undulations on a racetrack though so stiffening the forks with 40 weight may be a great idea for that type of riding.
On my Victory I upgraded the front end with a thorough clean out (they tend to collect a lot of gunk at bottom of forks) drilling and chamfering the damper rods as well as fitting Gold Valves and Progressive springs weight rated a 10kgs heavier than my actual weight.
New seals too of course...I did accidentally make the spacers above the forks too long as I didnt think of the cap recessing into the thread.
This made my modded forks way too hard, just over the top, on dissasembly I figured out my mistake.
Forks worked really well for hard riding, though earlier this year I scored a great deal on brand new 2011 Kingpin front end , chrome and complete Upside Down forks, They handle just as good stock as my modded standard forks yet have a more supple ride over bumps, the best of both worlds, bonus is they look really good too, combined with a valanced Kingpin front fender and wide 18" Hammer wheel, in place of the skinny 21"er
The old upgraded entire front may make it onto the XS650, only time will tell.