Well the nitrogen will expand some but its nearley un noticeable. The biggest advangete i see in running the nitrogen is to warm up the tires on the start (i never do)
2 ways to look at that
1, Warming up the tire before the start leaves you with hot sticky rubber thus hooking up good. Also making the tire consistant in the race.
2, Warming the tire causes increased pressure thus leaving you with a higher inflated tire that wont grab as much,
I personaly dont heat them, IF i ran nitrogen I WOULD,
RB that is something i completely forgot about. A tank of nitrogen from a welding shop woudnt bee too much and last you a LONG time, 2000 psi filling little things like we run would take a while to use up.