Hi, do you mean 'Bowden' extruders, or maybe more like 'Flexidrive'?
Bowden has the drawback that it needs more retract since the tube makes the filament more 'flexible' as the tube always has to be slightly bigger than the filament there is a little space around it that causes this.
However I'm guessing you are talking about Flexidrive kind of solutions, of which I don't know anything! But here is the central topic for that: [
forums.reprap.org]
Good luck!
http://www.marinusdebeer.nl/