Well my first question would be, what are the laws governing the water in the creek? It may flow across your land, but it isn't yours. And even though you are just using it's falling to get energy from, there are some pretty draconian laws on the books governing waterflow. In many cases, you cannot place anything into the streambed that would impede the flow in any way, so the small weir at the top might be a problem. What may be allowed, such as is used on irrigation systems, is a pit/trunk alongside the stream, and the water you use flows into this pit and into your pipe. I would start with your local irrigation board/council. Your local fish and game and EPA will probably have a say also. Even if it is against the rules to use the water for this, if you can show them a non-interference system, they may be persuaded to allow your green power system.
Those efficiencies do sound low. AN AC generator should be higher than that. One of the biggest issues with hydro power is RPM/frequency regulation. The mechanism to do this with a purely AC system and be responsive to load changes is pretty complex. As a guess, I would say most micro hydro systems nowdays are generating DC and running this thru an inverter which takes care of all the regulation. DC with batteries also allows a pretty small water source to become viable as it can provide 24/7 and provide more total power than is consumed in a day, but be far less than the peak demand which the batteries cover for.
If this stream flows year round, I wouldn't bother with net metering either. I would go off grid... If that is possible, that may justify a little more investment in the system.
Ron