As Seb has Cisco "documented", the maximum throughput for a 4321 with a boost license is 1.5 Gbps. Basically, Cisco is removing the software performance cap, so you can obtain whatever the hardware, your configuration and traffic mix will allow. In other words, the router will work much as all the prior generation ISRs. Like those, it's unlikely your traffic with your configuration will obtain such a rate, possibly not even near it. Since the performance license (more or less) "guarantees" 100 Mbps, that's likely the "floor" of expected performance with the boost license and 1.5 Gbps the maximum possible. I suspect typical maximum performance more toward the floor than the maximum possible, but again, much depends on your configuration and your traffic mix.
I've attached a Cisco white paper on prior generation ISR performance. What to take note of is how much maximum performance can vary based on configuration and traffic mix.
