Bicycle Mechanics - Help: 9 spd MTB rear mech with 10 spd cassette

I've heard about using 9 speed MTB derailleurs with 10 speed cassettes. I need some help understanding this. Here's me situation: I currently have, on my brevet bike, 6700 derailleurs with 7800 down tube shifters. 10 speed. I have a 48/34 crank and I'd like change my cassette get some lower gearing, maybe a 32 or 34 large cog on the cassette. It's my understanding that the long cage 6700 derailleur will have trouble with this and that I can use a 9 speed XT or XTR and I will work great with my shifters indexing cable pull. Would I need to long cage mech or would the medium be ok? I do know that Shimano has different requirements for cable pull on the road and MTB derailleurs so I cannot use one of the new 10 speed MTB mechs.

Does anyone have experience with this set up? How does this work? Also consider that I may be changing to a 46 tooth big ring on my crank, in case chain wrap is an issue.

With the exception of old (pre-9-speed) Dura Ace and new (10-speed) Dyna Sys MTB, all modern Shimano indexed rear derailleurs and shifters use the same cable pull RATIO, between how much cable is pulled by the shifter and how far over the derailleur moves. So, the "speed" of the rear derailleur is not a factor.

You may be able to get away with a MTB GS (short/medium cage) RD if the capacity of your drivetrain doesn't exceed the wrap capacity of the GS RD, which I believe is 33T. A 46/34T x 11-32T drivetrain just barely makes this: [46+32]-[34+11]=33. So, this drivetrain would work, but if you went to a wider cassette (e,g, 11-32) and/or kept the 48T outer chainring, then you'd need an SGS (long cage).