[eeyore]

a grindy feeling when manual focussing shouldnt have anything to do with the converter - unless it was somehow preventing the camera from realising that AF was turned off but I have never had that issue with mine so maybe you just had a duff one.

with regard to autofocus the general rule of thumb is that it needs an effective max apperture of f5.6 to function correctly (though you can get away with f8 in bright light) this essentially means that with an f5.6 lens you can pretty much forget the 2xtc andcabn only use a 1.4 on bright days.

you could buy a short f4 or f2.8 lens and use these with converters but given the price you would be better off buying a longer f5.6 lens second hand - the 135-400s that jason pointed out could be good , or a second hand bigma or 400 prime , however when buying seconfd hand sigma lenses make sure that they function properly on a digital body as some of the older ones dont