IMHO, snort defaults in 6.7 protect against a very low number of threats. But security needs many layers, one more is always better. And you may have customized snort as I did.
To use snort with an high bandwidth link as yours, you need a lot of cpu power.
The tests made by @EddieA seem to prove that the number of rules is not affecting a lot bandwidth cutoff, but my old tests (with much lower bandwidth) told another story: rules number made a substantial difference.
I’m working on snort on 7, but I still haven’t tested performances. With a few rules and a weak cpu I can reach 30 mibt/s.