140mm rotors are for anorexic whippets who ride swinley only ;0). I'm suprised they stop you at all!
I'm 15 stone and ride Guisbrough/NYM, real men(tm) just grab the levers harder! I sometimes worry for my carbon levers and the Ti bolts holding everything together!
New rotor all the way, it'll cost you less in the long term than having to buy pads more often.
Avtualy, small rotor = more heat = more time bedded in, even in the shittiest parts of winter I can ride without worying about the pads, I've had the same set of organics in my singlespeed for 18months now and they show no signs of wearign out any time soon! The downside is they glaze over in the summer so occasionaly need the brake dragging whilst deliberately riding in mud.