Well the first things I would ask is
1. Who rebuilt it? My brother is a master mechanic , but never rebuilt a manual transmission before. He did his first transmission and it grinds like hell.
2. What fluid are you running? I recommend Pennzoil synchromesh.
3. what clutch setup are you running and is your clutch adjusted properly?
I had a rebuilt transmission by ultracare honda in my 85 si turbo running 11psi and never had an issue.
CG transmissions are bullet proof.. Something is wrong in the equation and I don't think its the "power"
BTW: Save some money and but the pennzoil. It is the exact same fluid from the same company as GM buys it. GM just costs a whole lot more .