heres mine,i got me a 996 throttle twist grip then bin the 996 cable as its too short,then with your 999 cable -i took it to a local cable maker who replaced the inner cable while threading the metal 996 housing adjuster,you want to retain the 999 cable as the outer cable has a plastic coat inside which make for a smooth action. i know of lots of 999s thet have had this done without any spring problems. failing that pro v twins can sort you a rs cable to replace the 999 cable for around £70-£90?. my rework cable set me back £10.
i've used a 999 and the throttle is to light and has too much travel, the single cable one was much better. And they still fit a single cable to the s4rs so carn't be too much wrong with it.
its the first "mod" i did on my 999,it had a better feel to it over the standard throttle,its not as loose as the standard. it feels just right as i have limited wrist movement.
Since having my 999r I have struggled with the throttle turn. After all the above and speaking to the guys at JHP I've decided to have one fitted while my bike is in for service. I'm picking it up on Saturday assuming the weather is ok and I'll let you all know what it's like. Hopefully a lot better!!!!