I really wouldn't say that you're asking for trouble installing OS-9 on a 6400, but I wouldn't expect blazing speed either. I personally use a UMAX C-500 240MHz system everyday, (which is roughly the same motherboard architecture and speed of 6400 and 6500 machines), and I haven't upgraded to OS-9 yet. 8.6 runs pokey enough (compared to it running on my iMac) on this machines, so I don't feel the need to slow things down further.
Added features generally = more slowdowns. Most of the time it's justifiable, but that's an individual decision.