I think you can use fw samp to do what you want, see:
sk112454: How to configure Rate Limiting rules for DoS Mitigation
Using this command you can set bandwidth/connection quotas that are efficiently enforced by SecureXL; this mechanism is vastly preferred to the Network Quota IPS signature which kills practically all SecureXL acceleration on the gateway. Unfortunately you cannot directly specify a certain interface for enforcement in the matching criteria, but hopefully you could do the same thing with carefully selected source and/or destination networks in your fw samp statement.
New 2-day Live "Max Power" Series Course Now Available:
"Gateway Performance Optimization R81.20" at maxpowerfirewalls.com