30mpg EPA Highway isn't efficient it is average. 35mpg EPA Combined is just crazy for anything short of a tiny car or a 6spd hybrid(The 6spd CR-Z only got a score of 34mpg combined). 25mpg combined vs 35mpg combined is a huge difference. 30mpg Combined is what most compact cars get but on Regular grade with 5spds.
The Silvia was 2750lbs before it died off in the US around 2002 with similar dimensions to the FT-86. It is not likely the FT-86 will be lower than that even with 100lbs off the engine weight(KA was 360lbs or so). 2800lbs sounds about right.
25mpg combined EPA would be par for the course. And this isn't the new Civic Si. Nor is it the New Supra. Nor is it the New WRX Coupe. You all need more realistic expectations IMHO

. It's bad enough that you are expecting something besides a Hachi-Roku successor when they have practically named it the Hachi-Roku successor.