Hmm, I only have a sigma dg500 super and a 540 ex or is it ez, anyway the film one, neither capable of hss, though I would like to be able to do second curtain off camera, so I assume from the above the 603 cannot accomodate that then?
YN622 is only 100% operational with Canon Type II guns, eg 580EX MkII and Yongnuos, so you may not get much more than basic manual triggering anyway. In which case, TBH I'd probably just get 602 or 603 for now, unless you plan to upgrade your flashguns some time.