I have thrown size 2 woolies with my 4wt...but the impact rivals that of a meteor. Make sure you use a heavier tippet with the really big stuff, and you may find it beneficial to shorten your leader length also. Generally speaking, I have ended up slinging the big woolies when I find myself in warmwater with my coldwater rod. IMHO, most warmwater species (bass, catfish, etc) don't require near as long or slight of a tippet as do the salmonoids. I also find that while bass fishing, I'm often trying to sling my fly up under branches or into thick cover. I think having a shorter, heavier leader helps to control the big projectile a little better.

PS-Don't worry about how far you can cast, its how far you can cast with accuracy that matters.