Canon 100-400 IS USM & Kenko Pro 300 DG/DGX 1.4

Hi
Anyone had experience of the 100-400L used with a Kenko Teleconverter?
Is it any good?

I've read that unlike the Canon converter it doesn't dissable the AF without a tape-up job on the conectors.
Any idea which Kenko converter is right for a Canon 550D?

For me the ideal would be the new 70-300L with the possibility of occassional use of converter but things are looking like it wont work (at least with the Canon Teleconverter)
--
Mark