Don't worry about being light. Because you are trying to go downwind as deeply and quickly as possible you don't need to maximize lateral resistance with your weight.

You can fix the rudder cavitation issues if when the puff comes, the crew first eases the sheet then sheets in. Ideally its a 4-6 inch ease followed by sheeting on until the shape is right. This allows the spi to open up to the gust and curls the leading edge a little more. Once the crew sheets on again, the boat will be essentially pushed downwind by the action of the spi flattening back out. The helm will actually feel a pull downwind. The reason is the boat will accelerate slightly because of the increased wind velocity and the sheeting on, which will shift the apparent wind forward. The skipper then turns down in response, sailing a deeper course at the same or a slightly faster speed.

The only other way to smooth out the ride is anticipation. If the gust hits before the skipper turns down and the crew eases then the boat will become overpowered. The goal is to ease the spi slightly at the same time the gust is just starting. Then sheet on as the gust hits. Then as the skipper feels the acceleration and apparent wind shift turn down. This way the boat is constantly in sync with the increasing breeze.

As the puff ends, ease the sheet and turn up until the boat accelerates. Repeat.

Hope this helps. Now i want to go sailing.