Latitude XT3 CPU options?

Does anyone know offhand what CPU's are supported by the XT3's bios? I'm mostly interested in the i7 options.

Most i7 configurations use the 2620M, but ive seen a few using the 2720QM.

When I check wikipedia for more information about "i7-6x0M" processors, the ones listed on the spec sheet for the xt3, I get even more options.

What is probably my best bang for the buck option? Or is it even really worth upgrading the CPU past the stock i5?