you don't loose anything at all running firewire 400 devices in a firewire 800 port. It's just a cable at the end of the day and they are backwards compatible.
As Rounik mentioned, it kind of depends on how many tracks you are trying to record, and playback/ stream (including samplers and samples) at the same time.
FWIW, firewire 400 can pass through 34MB ish a sec sustained. Better than USB2 for sustained, not peak transfer.
As a point of reference, I'm working on a friends iMac, old type not spanky new silver ones, everything is off the internal drive and we have around 100 tracks running at the same time. 20 of those are software instruments, the rest audio. I am still able to record 3 tracks at a time on top of that without any issues. 512 buffer setting, not many plugs on top of that though apart from compressors where necessary and a reference reverb for the singer.
If in doubt, try it!