The reason that the slide throttle are hard to modulate is because the intake runner area increases/decreases greatly at some positions. Friction shouldn't be much more than a conventional throttle if the slide throttle is designed properly with bearings but maintenance/greasing is much more important. I'll see if I can find a good photo of one design.
Edit: Found a page but only thumbnail photos, the bigger ones does not seem to work? One good pic on the frontpage though http://www.scheel.ch/ its under "Motorsportteile".
I know this is an old thread, but in case anyone stumbles on this with an itch for slides...
I had a '90 Yamaha FZR1000 that had slide throttles. I can't remember what size the mikunis were, but considering they were designed to flow at 11.5k RPM, they should be sufficient.
They were carbs, but with a little JB weld and a dremel, probably wouldn't take much to make them suited for fuel injection.
They operated smoothly, and I really didn't notice any negatives at partial throttle. But if you're building a track car, it really doesn't matter.