Les

At least you get something in return from your panels - Zuma is just a liability. So adding panels does work. Your inverters should handle more than are rated for. It is fairly common practice to over-size one's PV array by up to 60% in other parts of the world. This means that your SCC is working closer to its maximum capacity for a larger part of the day. This is fairly common place in Aussie and the States. So long as the voltage and current are not exceeded you should be fine. The SCC will back off to protect itself. There is a guy in Papua New Guinea who is on the Aussies AEVA site who regularly does this with Axperts. When my own system was originally mooted I was going to an Imeon 3.6 with 4.5 kW of panels. You are right in that your efficiency goes to pot.
Read up about it. My next spend on solar will be to over-size the array but I am a long way off from that. The drought we are experiencing has wiped out our cashflow and the bank mangers usually friendly demeanour has disappeared (In fact the bank manager has disappeared to be replaced by one who is a lot less understanding. I think it is a strategy to replace bank managers in bad times in rural areas as the new bank manger listens to head office whilst one that has been there for a while is more sympathetic to the client's predicament).