Cisco VPN :: Max IPSec VPN Throughput On C1812 And C2811?

recently we had some performance issues with C2811 which caused us to do some lab testing. For testing we used also C1812. The results were quite surprising for us, as the C1812 appeared to be more efficient than C2811. Below you can see the lab scenario and results.

1. Why C2811 is performing worse than C1812?

2. Is there any official Cisco reference stating what are the max VPN throughputs of certain platforms/models? (we consider migration to C2900 platform and would like to choose the right model)

[URL]

as presented on the small diag:

All routers had enabled onboard hw VPN modules and SEC/K9 IOS ver. Configuration was very simple and beside encryption there were also GRE tunnels configured and EIGRP process for routing between "remote LANs". Part of conf responsible for encryption:

crypto isakmp policy 10 encr aes 256 authentication pre-share group 5 lifetime 3600crypto isakmp key ......... address ......... no-xauth!crypto ipsec transform-set SHA-AES256 esp-aes 256 esp-sha-hmac
crypto map VPN 90 ipsec-isakmp set peer ......... set transform-set SHA-AES256 set pfs group5 match address .........
 
TEST RESULTS
                                                                                                                                                                                                                       Cisco 1812Cisco   2811iperf   generated BW [bps]WAN   if BW (max of 30s avgs) [bps]CPU   usage (max of 5s avgs)WAN   if BW (max of 30s avgs) [bps]CPU   usage (max of 5s avgs)500k--540k5%1M1,1M3%1,2M8%2M2,1M4%2,3M14%5M5,4M10%5,7M34%10M10,6M20%11,5M65%15M15,8M28%17M96%16M--17,2M99%25M27M48%--35M38M64%--45M48,2M72%--53M60,8M88%--59M67M94%--61M72M97%--