Is it possible to test a NIC latency by looping ports together? Can it be done at least in theory? Will ethernet work between 2 ports of the same card? or between NIC port and LOM port?
preferably on Windows 2008 R2
You can loop them, but be sure you don't have setup like NIC teaming or something else.
All modern NIC can be connected using straight cable, but if it's oldest one you need to use cross cable.
Before this, check manufacturer site, most of them has tools (like Intel, Broadcom) which can make loopback test (and lot of others) without any special conditions like this.
In real, I don't think that you can measure useful values when you make loop like nullmodem on 2 local ethernet ports.
Latency is basically caused by used bandwidth/traffic on L2 switches, so making local loopback test makes no sense for latency measurement.