In general Cisco does not do extensive performance tests for GRE/ IPsec without a VPN Card in the Router.
We focus on providing accurate performance IPSec data for our Routers when they have a VPN Module installed. We also provide VPN Router bundles that enable customer to purchase Routers with VPN Modules at discounted prices
The 1751 with a VPN Module does about 3.50 Mbps at 256-byte packet and 10 Mbps at 1400 byte packet
The 2621XM with a VPN Module does about 3.56 at 256-byte packet and 14 Mbps for 1400- byte Packet
This Data is posted on cisco.com at:
http://www.cisco.com/en/US/products/hw/routers/ps259/products_data_sheet09186a00800921d5.html