Thanks mmcartalk I have learned a lot from your excellent and informative response.
Anytime. Glad it helped.
In real terms the difference is insignificant I know but the surprise to me was that the RC200t was going to be "slower" than the NX200t. About a year ago I read reports on Torquenews, who speculated, based on their tests and expectations, that the IS200t (when it came out which has now been announced) as well as the RC would be a bit over 250 bhp and have a 0-62 km/h of around 6 to 6.5 secs. Now that would make sense to me given the aerodynamic differences in body style between the NX and RC. Plus the 8 speed gearbox etc. We bought a NX200t partly for me to assess the new turbo engine in the hope that I could look forward to my personal next change to a RC200t but these figures don't give me much sense of desirability. Oh and btw I am not impressed by the fuel consumption on the NX. Despite economic claims for this turbo engine I am getting barely 27 mpg (UK) on a 300 mile round trip. On the same journey, different weekend I got 29 mpg out of my ISF. Now that's impressive!
One thing that certainly helps gas mileage on the IS-F, compared to the NX200T, is the F's 8-speed automatic, which really gears the engine RPM down at higher cruise speeds. Unless it gets to the point where an engine is lugging, less RPM usually beans better mileage. And the IS-F's V8 has so much torque that it can lope along at those low speeds in very high gears, helping economy without much strain. That's probably why you managed to get 29 MPG out of it. In contrast, I myself had an IS300 with the old 3.0L straight six, which was barely half the power of the IS-F. With a 5-speed automatic (as opposed to your 8-speed), and only one overdrive gear on it, the low-to-mid 20s was just about all it could manage , even featherfooting it on the highway (it was EPA-rated at 23/17).
One question, though.....to make it a little clearer: Was the 29 MPG you achieved with the IS-F (as on the NX200t) with British Imperial gallons? That would make the mileage a little lower (maybe 25 MPG) with American gallons.
And back to the NX200t, its mileage ratings are a little unusual compared to most car-based AWD vehicles. Most vehicles of this type do about 1-2 MPG better for all conditions (in American gallons) in their FWD versions, but the NX200t gets the same EPA city/highway ratings in both versions, with the FWD being 1 MPG better combined.