A vx/cx tranny will be better for fuel economy, and it will help a lot. I'd say probably around 10%, depending on your driving situation.

an hx, cx, or vx transmission will make the car sick dog slow, but it will probably bump the fuel economy by a good few mpg.

after you do the swap its important to remember that the car is not what it was before, if you wind the gears out hard trying to make it accelerate like it used to then the mpg will not improve as much.

this is all hypothetical though, you havent bought the car yet.

if i were to get a del slo s for fuel economy i would leave the motor alone aside from anal retentive maintenance and focus solely on reducing rolling resistance and doing something crazy like what basjoos did for aerodynamics. i believe that the del sol has a much smaller frontal area than a civic or even integra and even just a boat tail and belly pan would do more than enough.