Mountain Biking - Ultegra cassette with Sram x.9 twisters/derailleurs?

Is this do-able, and if so, is there a recommended Sram 9-speed chain to help marry up the shimano cassette to the Sram x.9 rear derailleur? There appears to be five different Sram 9-speed chains on the market, so would one chain be preferred ... if ... you can use a Sram derailleur with a Shimano cassette?

Yes, you can. It is actually very common for weight weenies to run a 12-27 as it drops a significant amount of weight even compared to xtr or the pg990. The problem is that it requires you to ride in the small ring more often, requiring more front der shifts, which increase the likelihood of dropped chains/chainsuck (yes, even if everything is adjusted correctly). Personally, I would rather run a double front and 12-32 rear because shifting up front is nowhere near as fast as shifting the rear. It's not as light, but more practical for me.

If I lived in a flat area, I would run double 28 or 29/42 and 12/27. If you live in a flat area, you may not need the granny ring anyway, so give the road cassette a try :)

I used to run a 11-21 DA CS7700 cassette/Shimano HG-93 chain with an X.O gripshift/derailleur setup and a single 38t ring up front on the DH rig and my XC bikes had 12-25 DA CS7700 cassettes/HG-93 chain and a 22/32/bash ring up front and X.O gripshift. My current ride also has a mix of SRAM/Shimano components.

If you have not bought the derailleur yet and want to stay with a Shimano group but like the gripshift then you could just get a set of SRAM Attack or Rocket gripshifters since they work with the Shimano indexing.

DBD:beer:

You have to strong to get up a 10% hill with road gearing. Compact double crank with 50/34 and a rear cassette of 12/34 is just about doable on our more gentle hills- but Full 44/34/22 and 11/32 works for me on our hills- so why change?