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.