With the achro, you will get blue bloating of the stars, even with an LRGB method and separate focusing of each of the colour channels. The achro just smears the blue light too far across the spectrum.
I've tried it here with a SW120 f/5 achro, and the result is not great:
http://www.astrobin.com/125603/
However, as noted above, if you use NB filters, you can get really nice results with an achromat, e.g. my shot of Helix
http://www.astrobin.com/135036/D/ or Skull
http://www.astrobin.com/125368/ -- both done with an achro at f/4