I'm also a Prelude (gen 4) owner and I have been doing some FE experiments the last months. I have the 2 liter engine with manual gearbox.
The easiest thing you can do is to tuck in the side mirrors. This seems to have a dramatic effect on aerodynamics and improved FE by about 5% at highway speeds for me.
I also switched to 195/60 16" wheels which is a 5% increase in circumference. This increased FE by another aproximate 5%.
I currently get about 40 mpg at highway speeds with a bit of Pulse & Glide and other tricks. However, this is in the Swedish winter with quite a lot of rain, snow and wind. I expect figures to improve when it gets warmer.
I have noticed from looking at raindrops that the airflow isn't attached on the rear window, which is a little surprising looking at the shape of the car. I haven't done tuft-testing yet but I suspect this has something to do with the turbulence from the seals around the sunroof.
I'm going to do the yarn testing this weekend, i'll keep you updated. i already "fish-scaled" my front fenders as much as possible. i haven't driven on the highway though, so i can't talk about the noise. i hope between the two of us, we can come up with something to net a total 15-20% gains. 40 is really impressive, i have an 2.2L-auto and the best i've been able to get is about 26-27mpg.
26 mpg highway?!?! What speed? I thought Hondas were supposed to get great economy. My 3.0L, LeBaron sedan (0.41Cd) did better than that stock with an automatic. If that was my car, I would be looking to fix it first before doing any aero mods. Is your gearing just that short (high rpm's hurt FE) or maybe you should consider a tuneup.
You don't necessarily need a race-style diffuser, but most cars have a big gap between the rear bumper cover and the trunk well. Consider filling this area in with something. Keeping air from catching the rear bumper and turning it into a sail can't hurt.