The mountain bike RD's I have been looking at, for the most part say they are compatible with 7 and 8 cog casettes. Nothing about six. Doesnt seem that a six would make any differance either as far as I can see?

Looks like my cheapest option would be a mountain bike RD change, 34T large cog. Going to pencil out the gear ratio's, but Im not liking what I see with only a 6T on the back. Not sure there is enough differance in changing out the Crank to a compact 50/34, compared to my 52/38.

I use my bike as a dailey commuter, and then will use it on the supported tour.