Najeem,
L2TP is used for tunneling wireless traffic (or any traffic really) over the wired network to some other destination. For a wireless point to point link you will need to setup an AP mesh between the two different APs. The following how-to guide explains the procedure (the guide is written for 5.1 software so some commands may look a bit different in other software releases):
http://www.michaelfmcnamara.com/files/motorola/WiNG5_Mesh_How-To.pdf
As for performance, this is complicated and is heavily dependent on the quality of the wireless link and the RF environment surrounding the site. The overall throughput of the link should be approximately equal to the average wireless data rate of the point to point link divided by two (half duplex communication) minus the protocol overhead.