These are very simple carburetors - once you get familiar with them. There really isn't much to them.
Here's my 'take' on your situation:
Some guys like to replace everything and know it's all new. I tend to replace what's worn out and use up what aint broke. That is, unless it's deep inside the engine or transmission and could cause a lot of trouble (and has a history of breaking).
I've found the butterfly o-ring lasts a very long time. It's protected inside the carb from oils, fuel, and most things that would damage it. If there's a problem with high idle (air leak) and everything else has been crossed off the list, then do em. But otherwise I wouldn't bother.
Diaphrams - that's another item easily replaced and easy to diagnose. I wouldn't worry about them either.
If the choke works properly, the bike won't start cold without it, but does fine with, then cross it off the list.
Jets don't wear - but they do clog up. Especially from fuel drying up in them and leaving a gummy varnish. But since your bike has been run regularly, and there have been no complaints about flat spots or other lean conditions, all seems fine in that area as well.
Treated properly, these carbs don't seem to wear out. I love em! But...
The needle rides in the needle jet and if you've ever taken a good look at it, there's a lot of play! It flops all around and they contact each other. The needle is aluminum, the jet - brass - both fairly soft metals and they do wear. Your fuel consumption tells me those two items are letting waaaay too much fuel past. And there's where your performance has gone - an over-rich mixture makes for a sluggish bike!
I'm not saying some good could come of a total clean-up of the carbs, but it's not necessarily the best bang for the buck. I believe all that's needed is the needles, needle jets, and perhaps floats. There are new black plastic floats to replace the old white foam ones - yours are probably a dark yellow or brown by now. The floats tend to get heavier over time which would cause the fuel level in the float bowl to ride higher than spec. This would also cause a rich mixture, reducing performance and mileage.
The best (and cheapest) place for BING parts is from the BMW dealer. The black plastic floats - I think they're not even on the parts list at BING, so you'll have to ask for them specifically.
