Well, this being caused by M_opt being larger is more plausible than it being caused by M_ship being smaller, for sureYou can see another consequence of this in action. Supercharge yourself, then make a jump equal to your normal (unsupercharged) maximum range. Observe how the fuel consumed is much lower.
However, while I doubt we'll ever see the relevant source code, I'd be willing to bet that it doesn't tweak M_opt temporarily. (Probably it just uses a separate FSD-boost multiplier which gets reset to 1 after each jump...)