Is it not obvious why he wants a sixth gear?
I don't think he is concerned with speed...

It's called gas mileage on the highway...

Extra gear = less rpms = less gas used

As for the speed:
Just adding an extra sixth gear to your current tranny, will not increase acceleration... It may increase top speed, but only if the engine has a strong torque curve in the lower rpm range (typical of large-displacement engines, aka less efficient engines), as is NOT the case with the prelude... Torque peak is at 5250, so shifting into a gear at a lower rpm than this will not add any "extra" top speed... The car may actually slow down to the speed at which the lower torque rating is actually able to keep constant speed...

A "real" 6 speed tranny will have shorter gears throughout, which translates into a better mechanical advantage for the engine, and therefore = better acceleration, WHILE IN GEAR (terms of G's)... Don't forget there is an "extra" gear change that takes time to complete...

But I don't think the tranny swap is even viable...
I agree, u want a sixth gear, buy an S2k.