Dave,
Agreed, we've known that for thirty years or more. Sikorsky tried, spent lots and then gave up.
Still ultimately limited by the single rotor, but how about the revival of the "rotodyne" principle? Thrust engines, tip jet start and finish, with autorotation in the middle. Low blade pitch angle in the cruise gave it a good speed range coupled with excellent load lifting capabilities.
The very fine Fairey Rotodyne was killed off, mainly on politics over noise issues, over forty years ago, but is generally recognised to be way in advance of its time.
Surely it should now be possible to design a quieter tip jet. These aircraft even get complimentary blade de-icing into the bargain due to the ducting of compressor air! After this winter in UK, that's a great bonus.
P.s. Doesn't the Fairey Rotodyne still hold the speed record for a rotary aircraft flying between London and Paris?
Here is an excellent link for those not familiar with that aircraft:
http://avia.russian.ee/vertigo/fairey_rotodyne-r.html