They're Flat Slides, not sides. They'll give you increased top end once they're set up properlly (maybe as much as 10hp) but you'll lose all the fuelling below 4000RPM - might make the bike a handful if you're still planning to ride it on the street. You might also want to look at getting a gear driven cam conversion for it.
10 hp, not really. They will give you better throttle response than CV carbs, which most bike have. However they are tricky to tune and you must have an accelerator pump with them. With flatslides you control the opening of the slide, not the engine vacuum. A few more hp, but not ten..
I use to have some flatslides on my ZX-6. They were set up great. There wasn't any real degradation in rideability around town. I just crank the throttle wide open at low speeds. The Flatsides may give you a few up top but they also give you harder acceleration, and once your rolling, it's instant throttle response.