2.5" S-ATA HDDS with 5400RPM can deliver up to 75-80 MB/s, 2.5" S-ATA HDDS with 7200RPM can get up to 95-105 MB/s, Firewire 800 can get up to 75MB/s, but stays more around 65MB/s, thus my conclusion, that a 5400RPM HDD will be okay with FW800.
Or are you talking about 3.5" HDDs?
Btw, the bigger the HDD capacity on small HDDs, the faster they are, as a 320GB 2.5" HDD with 5400RPM isn't as dense as a 500GB or 750GB 2.5" HDD with 5400RPM.