Re: Fly size and castability
Yeah, 2 or 4lb falls into the category of "light". Bumping up to 8lb won't make the fish fight less, as its still attached to the same limber rod. Your casts should turnover better though, and you'll have less chance of leaving the fly in a fish.
You may also find that you're comfortable casting closer to overhanging trees, etc, since you'l have a better chance of pulling your fly out. I'd be willing to bet you'll be amazed at how many more fish you can catch when you're casting closer to the cover. We've got some limestone bottomed streams here in Texas where you just cast under the trees and ignore the rest of the river, as the fish are all under the trees. (all the panfish, that is)