I had the same question, so I loaded up my car with about 620 lbs of passenger weight. It went just fine. It stays in a lower gear longer, but it adapts to the load intelligently. I don't know about the weight limit of the engine cover though. Loading it down with an extra full-size passenger makes the suspension and drivetrain respond like any other car with a full cargo load: slower takeoff (just give it more gas) and braking (brake earlier).
So, in short, it was fine for short trips but I wouldn't exceed the limit on a regular basis. An extra 300 lbs in a car this light is like adding 600 lbs of bricks to your average sedan.
In your case, 600 lbs of cargo plus the driver would be a total weight of about 750-850 lbs... I think the engine and transmission can handle it, but it may test the stability.