What is the limit for firewall rules?
There is no set limit.
From your experience when should we expect performance to be affected?
This would depend on many variable factors, so there is no set expections, however, you should create the firewall rules in an optimized method. Group up firewall rules were possible (HIPS 8 allows you to specify multiple objects, like IP addresses, applications, etc. in a single firewall rule; HIPS 7.0 does not).
Is there a way to import a list of IPs into a firewall rule (as opposed to single IP and range)?
There is no import functionality for this.