Yes, there are just two things. One, I hope we can be clear that we're talking about GM in agriculture and not industrial biotechnology. That's very important here.
Two, there's a good paper by Thijs Tollenaar and Liz Lee put out on cross-science in 2007 that is a detailed analysis of yield increase in corn over the last 100 years. Thijs currently works for Monsanto in the RTP and was formerly a professor in corn physiology here at Guelph. I can get you that paper if you want. It points out and backs up Manish's assertion that it's a polygenic issue.