The TS-E 24 II (unofficially) takes the 1.4x and 2x III extenders and they work very well. The sharpness lost is minimal at f/8 and beyond with the
1.4x III and little worse but still very usable with the
2x III. It takes a bit more of a hit as you increase shift and tilt, but still produces very good results even at say a shift of +4 and a tilt of 1.5. I frequently use the extenders with this lens and love the flexibility they add when you can't get closer - like when a river is in the way - this is with the 5DIII, TS-E 24 f/3.5 II + 2x III,
tilt +6, shift +1, ND6 @ f/16, 37s, ISO 100:
and a 100% crop - keeping in mind that the photo was degraded somewhat by high humidity and the wind during the long exposure. I have printed this at 20x30" and it looks very good: