The higher the number of sprocket teeth, the more stress on the transmission gears and case. A belt drive can change the ratio at the primary which can give the desired lower overall ratio without significantly increasing the stress on the transmission.
It is generally thought that due to the increased torque of the engine, the 850 can perform better with 21 and 22 tooth sprockets but a 750 will struggle and a noticeable drop in acceleration will result.
I currently have stock 750's with 19 and 20 tooth sprockets, a warmed over 850 with a 21 and a stock 850 (Interstate) with a 22. The 19 is not suited for more than a short blast down the freeway, where the 20 is somewhat better without sacrificing much acceleration. The 21 on the 850 cafe racer is quite good. A 22 would help top speed, but with the cam, carbs, and head work would probably be a pain around town and require slipping the clutch on normal starts. The 22 on the Interstate (stock 30mm head with a 2S grind cam) is quite nice on the freeway, still does well around town, but is certainly no jackrabbit on acceleration. Launching requires just a little more clutch slip, but not objectionable. All of these bikes have the stock chain primary. After more than 10 years, the box with the 22 tooth sprocket is stil intact with no problems, but I don't chase Gixxers on it either.