Fuel economy and emissions are very different.
The emissions issue is that a GT-R that is R34 and older doesn't have a catalytic converter (mandated on all cars sold in U.S. after model year 1975) and evaporative emissions systems while applying to OBD-I (after m.y. 1986) or OBD-II (after m.y. 1995) standards, which are federally mandated to be on all road-going cars sold in the U.S. R34s (and older) can be legally imported into the U.S., they just can't be driven anywhere other than at a race track without having those emissions systems installed. And that's where the problem lies, as it is easy to install a catalytic converter, but not so easy to install the other emissions systems. And what's worse is that cars that are retro fitted with emissions systems have to be fitted with the most modern systems, so there have to be pre and post-cat lamda (O2) sensors, evap systems, EGR (exhaust gas recirculation) systems, etc; and when all these systems are installed the vehicle has to meet California emissions standards (the strictest in the U.S.)
The safety standards are another hurdle, since the DOT requires that vehicles sold in the U.S. be tested on (I believe) 27 different types of crashes. This means that a minimum of 27 vehicles have to be crashed for testing purposes. And all those vehicles have to be nearly identically equipped when it comes to drivetrains (diff engines or transmissions = same tests, AWD vs RWD= diff tests, 2 door vs 4 door= diff tests). That's all not necessary if the manufacturer does these tests (meeting DOT standards) in their own labs, but anything not DOT approved has to be tested by the DOT before it can be legally imported as a road going car into the U.S.(and we all know how fast gov't branches gets things done).