From what I've found out the 1.4TC MIII has minimal optical gain over the MII with the greatest gain in the corners (if you're shooting crop sensor you'll hardly notice this, whilst fullframe will show this up more). Furthermore they are chipped to improve AF performance, but only with select MII L lenses (basically your 300mm f2.8 MII and longer lenses - the 70-200mm f2.8 MII is not supported).