Genetically modified canola was not bred to be yield enhancing, but herbicide resistant so that farmers could improve their weed control. Canola yields will vary annually according to weather patterns, as is the case for any commodity, whether it is GM or not.
The Canola Council of Canada (CCC) has some very good statistical data on canola production (http://www.canolacouncil.org/markets-stats/statistics/).
For the 15 year period from 1996-2010, the CCC provides estimates of the percentages of the various herbicide (HT) tolerant canola varieties and conventional canola. This include HT varieties that are GM and those that are developed by mutagenesis. By 2010, they stopped collecting this information as virtually all of the canola produced was HT.
Looking at the statistics for production in bushels per acre, it shows that prior to GM canola, yields ranged from 20-25 bushels per acre. Even the first 5 years of GM canola being available, yields didn’t change much, with a range of 21-28 bushels per acre. By 2004, the adoption of GM canola passed 70% and the impact of the technology starts to become more evident as yield ranges begin to shift upward, ranging from 27-40 bushels per acre between 2004 and 2014.
The statistics for tonnes harvested also reflects the increase in the production of canola, but also captures the yield increases as well. Production has increased from 3-4 million tonnes prior to GM canola to 15 million tonnes presently.
The one thing that has improved canola yields is not a GM technology, but is the application of hybrids to canola. Hybrid varieties have superior yield potential and greatly contributed to increasing canola yields beginning shortly after they were first introduced about 2003.