I believe the Americans (who else!) played with a huge range of different formats and propulsion systems in the 50s and 60s, including fitting jet engines to the sides of a Huey, with rapid results.
The effect of retreating blade stall was countered by the use of small wings on some machines (unloading the disc), which turned them into the compound category, but I believe they could top 300mph.
I'm sure that Nick Lappos will know more about these machines.
I'm not an engineer, but on the basis that the tail rotor is not really needed in forward flight, presumably the power that it drains from the engine is very little (as it will have only small loading). Any forward energy given by a rearward facing propellor would therefore have to be stolen from the main rotor, and I don't see why you would end up with any advantage. Perhaps someone with better understanding could enlighten me.