Ewan,
The typical articulated coax has rotor rpm limits very much like a single rotor helo, and has no exceptional advantage in low low rpm operations.
Beware, Russians generally set 100% as the autorotation upper limit, so that low cruise rpm can be 85%, low emergency limit much lower. These numbers look low relative to a helo that has 100% as its normal rpm, but the tip speed is not lower.
Stall affects the control system vastly by overloading the pitch links and servos, coax or not. This means the controls would have to be very heavy to support the post stall needs of your low rpm mission segment, a price no coax helo manufacturer has yet decided to pay.
Also, a coax would seem to have an advantage in stall because a fully lifting blade is available on both sides to keep cyclic control, but the stalled blade will wander and flap, and self-midair is a real problem. Word is the loss of the KA50 was due to maneuvering in cyclic, not zero
This is an example of how the ability to mix myths helps establish pop-aerodynamics - a teetering rotor flaps and removes itself at zero g, thus an articulated coax must have seen zero g to have a rotor problem. Monty Python had similar logic, if a witch floats, and wood floats, then witches must be made of wood......
The payload advantage that some subscribe to coax configs is not real, because the disk loading is the hover performance driver, and the two coax rotors weigh more than a single rotor, more than a tail rotor system weighs. Also, the wide separation between the two disks (to prevent noise complaints when the blades collide with each other) makes the cruise drag very high, usually about 10% higher than a single rotor. This makes the payload less at any significant range, since the gas burned robs payload. Coaxs are great at shipboard ops, where the wind can come from any direction without any practical limit, and short, stubby tail cones make deck spotting easy, and where mission ranges are miles, not hundreds of miles, so the fuel burn at cruise is a smaller segment of the mission.
The ABC coax concept that X2 uses resolves the post-stall flapping/blade contact issue to some extent by having very rigid rotors so that almost no flapping can be observed. Also, the rigid blades can be brought closer together, so that the drag is less than a conventional coax, but still higher than a single rotor. The KS-50 has two articulated systems, with one at 2 1/2% offset, the other around 7%. An ABC rotor has about 25% offset on both rotors.