If you fish with smaller line and still fill up the spool, you definitely run a greater risk of backlashing because there is more line on the reel. If you want to fish a smaller line, pack it on as tightly as poissible.
there should only be a slight diff. Try adding a little to the braking system, and make sure that you put the line on the reel very tightly. every once in a while, throw a long cast and retrieve line through your fingers to get it back on the spool snugly.
Yes 10 lb will nest up a little easier than 12 pound and 10 pound will break on any type of nick in the line. However I do like using 10 pound in the wintertime especially when fishing west bay in that gin clear water. I also like using it when wading because of the extra distance. I do change 10 pound test after every 2nd trip. I use nothing but Berkely big game for mono. If it aint broke dont fix it!!