Might as well call it a scam. Air is mostly nitrogen. The effusion phenomena is related to molecule weight, not size (Since O and N have the same number of orbital shells, they would be the same size) While N2 is slghtly lighter than O2 (28vs32) the difference is pretty negligable. If we were comparing oxygen to helium, the difference would be more sagnificant.
Furthermore, nitrogen, being a gas, obeys gas laws. Thus it increases pressure with temperature the same as any other gas.
There are however, some mild benefits from nitrogen. It is a stable gas, and will not corrode the rim (But yours is still powdercoated right? so it shouldnt make a difference). And nitrogen is dry, not carrying any moisture. The moisture in air well dew inside the tire, throwing the balance off slightly (and I do mean very, very slightly). Its a usefull trick in racing where every bit counts, but a complete waste of money for street/trackdays.
Even in racing, nitrogen is primarily used to run highspeed pneumatic tools, which do not tolorate moisture well (Cant have the impact wratchet fail in the middle of a tire change) Since its already there, why not put it in tires?