I have had issues with after market alts at 6-7000 RPM as the output drops to 12.0 or so.
After replacing the pulley with a stock one which gave better output, but still output drops over 6000 RPM.
After discussing this with an alt repair shop, I was informed these alts has a built-in limiter which drops off voltage at high RPM.
I use a volt booster which gives 14 volts at RPM, but I now use an aftermarket alt with a stock pulley and a life time warranty, as the alt last will only last a year or 2!