I also have a 400hp s15 spec r...
I haven't played around with the racing susp. kit yet, but with just the height/damper-adjustable coilovers even in their standard setting I find the s15 very controllable while drifting...
Just make sure to do the regular adjustments for drifting:
- n1/n1 works well
- drop suspension height by 20-30mm (varies depending on track elevation changes/ripple-strip dimensions)
- stiffen spring rates in line with height drop
- stiffen damper rates in line with spring rate
This works well with my style (enter corner slightly faster than grip-driving line, use handbrake/dirt-drop/small feint to initiate drift, couple with a sharp blip of throttle to really have the car sitting at a nice angle, balance with counter+throttle modulation)
If you have problems with initiating (too understeery) then its time to mess around with spring rate/damper/LSD F/R ratios...