FWIW, Chapter 9 Table 9 on AC conductor reactance and resistance lumps EMT and rigid steel conduit together. So whatever difference in electrical characteristics they have is below the precision of that Table.
This paper discusses some of the physics involved and does some testing. Primarily in the context of line to conduit faults, which compared to normal operation would accentuate the differences in the conduit, as the conduit is actually half of the circuit. I don't have the background to interpret the information in the paper or to come to a conclusion, but my slightly educated guess is that someone who does would reach the conclusion that they are close enough in electrical characteristics.
Cheers, Wayne
P.S. I guess a related question is what level of "sameness" is required by 310.10(G). If you have a 50' parallel run, and one conductor is 50' while another is 50' 6", does that violate 310.10(G)? If not, then 1% difference is negligible, which is is my guess for an upper bound on the EMT vs Rigid effect.
