Despite some concerns raised by the manufacturer's engineer, the wheels did not come off our newly reconfigured firewalls today. The box handling our Internet feed from AT&T chugged away without difficulty, and the box handling our feed from CEN did just fine. The CPU on that box spiked up into the high seventies during busy times, but it was just that -- spiking. Memory utilization was fine, and more importantly, performance was very good. Our peak utilization on that feed was ~40Mbps (as measured by a five-minute average -- I saw some momentary spikes up into the 65Mbps+ range).
I think our plan at this point is to let the boxes perk along separately for a while (read: a few weeks) and see if their good behavior continues. What comes after that depends on conversations I need to have with the manufacturer's support team.