I think it would take more than cams to make a 400ex keep up with any 450. But, it is a good start and like Bot said, it won't hurt reliability much, as long as you don't go with a cam with a really aggressive profile. If you aren't doing a bunch of other mods to go with it, you don't really want really aggressive cams anyway.
BTW, I changed the thread title.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.