Hello Awoog,
the ESW are lan switches modules you can install up to 2 but you need to cross-connect them to build a stack.
see
No more than two Ethernet Switch HWICs or network modules may be installed in a host router.
Multiple Ethernet Switch HWICs or network modules installed in a host router will not act independently of each other. They must be stacked, as they will not work at all otherwise.
http://www.cisco.com/en/US/docs/ios/lanswitch/configuration/guide/lsw_hwic_ethsw_ic_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1043334
L2 performance are not a problem but for providing L3 services you need to configure two SVIs associated to two different vlans.
L3 performance is that of the router you can find raw figures in the attached pdf
2851 is able of doing
220,000 pps 112.64 Mbps so it should be able to do the job.
Take in account that any additional features like NAT or QoS reduce the overall performance because it works in software only.
L2 ports when accessing other ports in same vlan are handled by HWIC without involving the main cpu.
Edit:
you may want to consider a multilayer switch if NAT is not needed
Hope to help
Giuseppe
