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First, in your referenced document, the performance is for a 3945, not the 3945e. The latter has about 3x the performance of the former.
In the attached Cisco white paper, Cisco recommends a 3945e for up to 350 Mpbs of WAN (i.e. duplex) bandwidth. However, if you look at table one, 64 byte PPS is listed as 2.924 Mpps, good for about 3 Gbps, and for 1500 byte packet bandwidth is listed as 8.675 Gbps. So, depending on your device configuration, and your traffic mix's packet sizes, your CPU usage isn't really unexpected.
To your question on how much more performance you might obtain. Well, again for you traffic mix and configuration, if you're peaking under 20%, in theory, maybe up to 5x your current traffic average. However, NM and EHWIC cards are noted as supporting 2 Gbps aggregate bandwidth, between module slots, across the MGF. That would preclude 10G modules, and the MGF, itself, would likely also become a bottleneck if you try to load the router up with additional gig ports.
