Yes, I currently use a Jarvis Custom P99 9mm threaded barrel and really love it. It was expensive however... expect to pay about $250 for the barrel. As far as whether or not it was "worth it"... hmmm, hard to say. $250 is a lot for a barrel, but ultimately, we P99 affeciandos don't have much of a choice.
Unless you can secure an actual Walther threaded barrel, I'd go with Jarvis. The quality/accuracy is amazing. Just two things to keep in mind with Jarvis:
1) It will take them 2-3 times longer than they say for you to get the barrel. I ordered a .40 P99 threaded barrel in late summer and they are just now getting around to finishing it this month. Walther barrels in particular seem to spend more then their fair share of time on the back burner. Still, it is definately worth the wait. And there is always a chance they may still have 9mm barrels left over from the last batch. Call and see.
2) Jarvis barrels tend to be a little tight at first and MUST be broken in before adding a suppressor (if that is what you had in mind) or they won't cycle. This means putting about 200 rounds through it without attachments. Once you do this though, it will function flawlessly.
Good luck and let us know what you decide.