All these mods really don't increase top speed. What they do is get you to top speed quicker and help keep you there. Peak horsepower hits at around 8200 RPM, the falls off from there. What I notice is at 8700 RPM (61 MPH), you run out of steam. Right now, a stock Z125 could theoretically outrun my modified bike, if they had a long enough downhill slope. My guess is a gearing change will help. I'll post back results when I find someone with a 15 tooth front sprocket in stock.
The other option would be to install a performance cam which would bring peak hp at a higher RPM. I'm not sold on the cam change as my intended use for the bike is blasting down curvy rural roads. I would prefer the motor run at lower revs as I plan to put a lot of miles on it.