I had a 1.6 Supercharged C180K Merc. The new models replaced the SC with a turbo and gained a few MPG combined, for the same power output.
Turbos are a bit better for fuel economy than supercharged, its only about 5% based on the Mercs but in todays market where a few hp matters, turbo wins.
You still get better fuel economy from a 200hp 1.6 supercharged than you do from a 200hp 2.0litre high revving N/A engine. Partly because the higher revving engine has shorter gearing to make use of the higher revs (to achieve the power).
My 86 is doing about 3k at 70mph. My 20d BMW is doing half that with it 8speed gearbox. Frictional losses for one are lower with less revs and as been said forced induction improves thermal efficiency of the engine.
Next gen 86 if it does go ahead will have to be turbo or supercharged to keep costs down I would think.