Adding 1.4x TC adds a bunch of extra lens elements to the optic pathway, so I prefer to add (hollow) extension tubes to my 100L Macro lens if I want more magnfication. The Pro-Optic extension tubes from Adorama are built well and cheap!
If you say "prefer" you have a tc on the macro, too? I just met a pro photog who only uses 1.4x and 2x tcs on his Nikon macro and wouldn't want it any other way.
I like the small tc because given the excellent iq of the 100L, the iq hit is near to unnoticeable, it works like a real 140mm lens at all focus distances (I'm dual-using the 100L for other shots, too) and the working distance for macro increases which is convenient for bugs and the like.
I'm thinking about getting a 2x tc for this macro lens, too - you could say I should get the 180mm macro, but the 100L is cheaper, has IS and the tc solution seems much more flexible to me, and vanilla external flashes work better with the tc solution.