You'd have to query the switches to find out the throughput they are undergoing.
From that you can put the bandwidth numbers and required blades into the Appliance Sizing Tool, at least as a starting point.
But, I would be careful replacing a "Core Router" with a "Core Firewall" as it creates a single point of failure in the environment.
I would use different firewalls depending on the tolerance for outages, throughput requirements, governance, segmentation, etc.
From there, you can come up with a design that meets the performance and security requirements.