I've never built an engine, don't have the knowledge to build an engine, don't have the inclination to build an engine. Mainly because it makes more sense to work backwards: until you minimize losses due to driving style, aero and rolling resistance, it doesn't matter what you've done to the engine because you're going to force it to work through all those inefficiencies.
At the right ambient temps, my engine is efficient enough that I get 300 mpg while gliding at 60 mph. If I want better than that, I've got a key that brings it up to infinity mpg. Call it a failing of mine, but I'm perfectly happy with that kind of efficiency under the hood. Maybe if I get really ambitious a WAI could bring that up a bit, but it'd take more testing than wrenching, and would only ever pay off if I saw it as a "look what I did" mod.
Tire pressure and a grille block, OTOH, produced big reductions in rolling and aero resistance, and the block actually lets my non-wasteful engine put some heat into the cabin in the winter. Another fun fact about aeromods is that they help even more when you're going faster. If it helps, I went high end with the grille block: coroplast instead of cardboard, recessed and painted black. When I posted a pic here, someone thought I had accidentally posted the "before" shot.