I've had a 900SS, a 916BP, and I've just gone back to a 900SL. I didn't do it because I love the engine, more because I love the style, low hassle factor, noise and the lack of financial/security worry!
Having said that, I don't really find the 900 that under powered. Long term I'd like to chuck a 1000DS lump in there with a 1080 kit from Sigma Performance, but that has more to do with low down grunt and getting the front up on what's quite a long bike.
In the end the figures speak for themselves. A 1000DS chucks out 85ish bhp, while most 999s make at least another 45% on top. That's big bananas on a bike weighing 190kg. For me though a 1080DS with about 92bhp and shed loads of torque would be prefect. Just got to find an engine now.....
Cheers,
Ali