I personally won't overline a rod either, i use a #6 for my 6 weight rods, and #7 for the 7 and so on, i use a Scientific Anglers sink tip, and use that probably most of the season (when dries are not the order of the day) or there is little surface action, the sink tip allows me to fish nymphs or buzzers below the surface, deeper than a floating line will, i find it good for searching the depth fish are feeding at. A cheaper option is poly leaders, which will basically act like a sink tip, the sink tip lines also come in a variety of lengths from 3foot to 15foot tips as far as i know, so it will depend how deep you want to be searching for fish, good luck.
As an aside, if you feel your casting presentation is lacking something, a casting lesson would probably work out cheaper than a shiny new line
Best regards
Jim