Yeah a 20 degree drop across the evaporator seems reasonable. That's more a function of the surface area of the heat exchanger itself. Keep in mind, however, that as the ambient air inside the house cools, that 20 degree drop will continue, *progressively* cooling the air more and more.

If the system has a leak, even on the low-pressure side, it won't maintain refrigerant. These are volatile compounds that evaporate readily to the atmosphere (making them good refrigerants!). Also, even a small leak will get water vapor in the system, totally screwing you.

I think you need a new evaporator and consequently a evacuation and recharge. Too early to tell 100%, but that's how it looks to me.

J
Edit: the fact that it cools below the dewpoint means that it's got some refrigerant, so possibly a full evacuation and recharge could be enough.