I'm looking at using a Kenko 2x Teleplus Pro 300 DGX with two specific lenses on my 5D Mk III:
70-200 f/2.8 II
100L f/2.8 Macro
Two variables: possibly using extension tubes with the teleconverter and the 100L, either on my 5Dm3, or on my 60D (more for composition purposes on the 60D, with the understanding if I do that I'll lose IQ over just cropping into a 5Dm3 shot - can't crop with my eyes when taking the shot though).
I've read a lot about issues with the 5D3 and kenko converters though. Blue dots, green dots, specific lenses, etc.
If anyone has recently purchased kenko teleconverters (the DGX 300) and is able to use them with all the above equipment (the 5d3 on latest firmware), please let me know.