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Ok, yes a 3845 is rated at about 500 Kpps, for minimum size packets, but that's also for "ideal" traffic. The same device is also rated at 35 Kpps for process switching, which provides a hint of the CPU hit for anything less than ideal traffic might cause. (Yes, I noted you don't see high process switching, but interrupt switch is still CPU processed, it's just, in theory, very optimal at what it does.)
Again, if your VPN traffic is just transit, I wouldn't expect it to normally have any more impact than any other similar sized transit traffic.
What you might try is some traffic generator. Shoot an additional 80 Mbps through your router, and see what's the CPU impact. (On Windows clients, I've used PCATTCP with UDP packets for such testing.)
