I ride an '00 M900 and rented a BMW 650GS for a couple of weeks in New Zealand. I can see where you're feeling the lack of oomph. That said, I thought the 650GS was a really nice bike, and really only missed the HP once I got over 110-120 KPH or when passing.
I have it on my to-do list to write up a model comparison for this board, but in the meantime here are some options:
If your heart is set on something new, then obviously you'll need to ride the 800, 1000 and S4 to see what feels right. You might also consider a Multistrada if the touring/upright riding/light dual sport thing still appeals to you. One thing to be aware of is that Ducs need minor service every 6000 miles and major service ever 12,000 miles. The 800, 1000 and Multistrada are all 2 valve per cylinder machines, while the S4 and S4R are 4 valve machines, with a correspondingly higher price for servicing. There are also fairly common problems with S4s and their batteries. You can slog through the posts on this board if you want the gory details.
If you're open to a used machine, you might consider a used 750 or 900. Either one will be faster than the GS. Again, you'll want to test ride to see. "S" models of either tend to have better componentry, while the Dark models tend to be more basic. "i.e." after the model indicates it is fuel injected, which many folks think is an improvement over carbed. The 750 has a wet clutch, the 900 a dry clutch and got fuel injection a year (or two?) earlier than the 750. Also the 750 and 900 have only one sparkplug per cylinder, the newer ones with "DS" in the name are dual spark plug per cylinder. Used 750s and 900s can be had for relatively little, although not quite the bargains they were immediately after the dotcom bubble popped.

If you get one that has about 6K or 12K on the clock, be sure to find out if the PO had the service done and deduct the cost when you negotiate if they haven't.
HTH