Impact of latitude on agriculture?
I know this response is coming a bit late but in case you're still curious I have a response!

I'm a M.Sc. student working in Agricultural entomology in Northern Canada (between 55 and 59 degrees North). You are correct, up at more Northern latitudes our short growing season mainly limits what crops we can grow, but less so our yield. We are also restricted in where we can farm by soil conditions as the further north you go the more you are on the Canadian Shield - so less fertile. If you look at growing regions in Northern Alberta, North West territories, etc. you'll see that they tend to stick fairly close to rivers. Further south in Canada, moisture conditions play a much bigger role in determining yield. Mainly we stick to crop species and cultivars that can produce a good yield in a short time - canola, wheat, barley, peas (although corn and soybeans have some large acreages in the Eastern prairies and Ontario).

Climate wise, the comment about light intensity is not really accurate in the summer. The longer daylength definitely compensates for any differences in light intensity. Even at 59 degrees latitude it can get well into the 30's (Celcius) in the summer. 

Canada already produces a large amount of wheat and canola, we are the 6th largest exporter of wheat according to a quick google search. So I would argue that while Canada may not be the bread basket of the world it is pretty close already.

It's tough to say how climate change will affect agriculture, because it's really tough to determine how climate change will affect local climate. The changes to weather patterns (changes in amount and timing of rain and moisture, changes in spring or fall frost timing) will have a large impact on agricultural outputs.