I have tried the Optical Flow setup on a Tarot 650 copter and was not happy with the results. There is a thread about it on the Developers forum.
I feel that the current M8N GPS system does a very good job for landing accuracy within 2-5 feet. That being said, the newer RTK GPS Receiver will be what you are looking for. Search the APM Wiki for "RTK". Today, you need a rather deep hobby budget to play with the RTK GPS Receiver but for commercial use it is very acceptable. I'm waiting for the prices to drop.