I think the best spacing for sweetcorn depends on your soil and cultivation conditions. If you can get them to grow really well, they will try to send out lots of side shoots and fully fill up the bed. However if conditions are not so good there will be embarrassing gaps.
For example in my back garden Sundance have always been best at 15 inches each way but when I tried my allotment I did better at 20 inches each way. I'm not sure the bigger plants gave a better total yield but I got bigger cobs over a longer cropping period.
I've since gone back to sweetcorn in the back garden because it is closer to the saucepan.