a) i am stupid and found where Loopshape has the "hasVertex" part (in getVertex). Tho i fail to understand it after that REALLY quick look i did.
a quick and admitably hacked test with the LoopShape seems to indicate tho that this is the culprit (i already had iterations up to get better Joint stiffness, and decreasing the timestep did not seem to change anything in that regard)
So thanks again! Will look at implementing ChainShape then i guess. From a quick look box2d actually implements both ChainShape AND LoopShape in the ChainShape source file, whereas box2d.xna and Farseer only have a LoopShape file that does not implement
I'll play with that and share my findings.