Even if you can install the 6-speed with the hydraulic clutch (valle and roadrunner, you must know something the rest of us don't [smiley=nixweiss.gif]), I would have to agree with bighd on this one. You've got to be running in excess of 70 mph to get the rpm's over 3 grand on these bikes. That is not an excessive rpm. Plus, the torque peak is actually higher on these bikes (with the stock cam, at least) than it is on a TC88 at around 4,000 rpm. So by pulling a sixth gear and dropping the engine speed by 500 rpm at highway speeds, you'll be putting yourself farther away from the engine's sweet spot which will result in less responsiveness (and downshifting) when accelerating. While you might get some increased fuel mileage with the 6-speed, it seems it would take lifetime of riding to pay back, wouldn't it?
Of course, if going to the 6-speed really does mean losing the fabulous hydraulic clutch, then that alone would not be worth the conversion to me.