I understand that the V-22 is capable of carrying external loads at pretty high speed and that is fine but the question is can the load itself survive it? A humvee is not meant to go 200 mph! In fact I recall some problems in testing the Army's new FMTV vehicle doing sling loads. The chinnok pilots were able to cruise along at a pretty good clip with the truck in tow....so fast that it would crack the windshields from wind forces against it. It was decided that the truck should be carried backwards so as to reduce the wind force on the front. This may work for a planned mission but I doubt the marines would want to remove all their tarp and bow kits and put braces on the windows and doors to keep from ruining their vehicles just so the V-22 can sling them into a hot LZ or beach at 200 knots. This would leave a bunch of Marines sitting on the beach trying to put their wheels back together while being shot at....not a good scenario in my mind.
From the numbers I have read the 609 and the 76 have similar useful loads but the power required is drastically different. You can get (or used ot be able to get) a PT6 powered S-76 which has about 980HP each. The PT6 in the 609 puts out I think somethig like 1680 HP each. I would assume that HP increase would be needed in the helicopter mode but I'm not sure how much less it will be for the airplane mode.
Max