Re. yields, the following is from a briefing on the IAASTD report, which is the most comprehensive that I am aware of:
"The IAASTD observed that the evidence regarding GMO impacts on yield is sparse, highly variable and mostly anecdotal. Yield declines have been consistently recorded in GM soybeans and maize, while yield gains have been reported in some situations and no yield effects at all in others. In many cases, yield benefits observed in GMOs derive from the developer?s use of high-yielding modern varieties developed over time through local and conven- tional breeding, rather than from the genetic engineering technology itself."
The briefing is here and the relevant quotes are on p5. The briefing is from the Pesticides Action Network in America, so not impartial, but the IAASTD report was a committee of experts and initially contained representatives from both Monsanto & Syngenta.
There's a useful commentary on the report here
"The IAASTD draft document is surprising for still another reason. Although supported by the World Bank, it does not offer much support for transgenic crops as the best hope, or even as a particularly useful tool, to alleviate the agricultural ills that beset developing countries, the hungry and the poor.
Most likely, inclusiveness and scarce support for GMOs by the IAASTD are in fact connected. It is probably no coincidence that a document arrived at transparently, using a tolerably democratic process (i.e. it was not written behind closed doors), and using a multidisciplinary approach, should conclude that GM crops have ?lingering safety concerns? and may even be harmful to rural development.
These conclusions in general, and the lack of support for GMOs in particular, are immensely unwelcome in some quarters. The publicity machines of Monsanto, Syngenta and others have not spent twenty years carefully positioning transgenics as the solution to every agricultural problem in order for them to be ignored by the largest and most diverse collection of agriculture and development policy experts ever assembled.
Last October, Monsanto and Syngenta resigned altogether from the IAASTD project. Though they gave no public reasons for their resignation, the industry body CropLife International told Nature magazine that an inability to make progress in arguing for GMOs was the fundamental reason (1)."