Interface speed of S200 and S400 appiances is 1 Gbit/s, for S500 appliances it is 10 Gbit/s (apparently meanwhile S400 have 10GBit/s adapters too). If you just tunnel through the traffic without doing anything to it, you should be able to transfer data at this speed. Of course you just buy a router then for a thousandth of the price.
For everything else it enirely depends on your policy. Is SSL interception enabled? How complex are your policy rules? How often do you authenticate connections? Do you use ICAP? What content filtering options do you use? How extensive is the logging? What percentage of the traffic needs to be decrypted and re-encrypted?
Symantec claims that a S500-20 is sufficient for an internet bandwidth of 1 GBit/s, but in my experience this is a optimistic assumption if you plan to use all features of this box.