Dan,
I think the 30 lb test is .011" diameter, roughly. The density is likely to be around 1/8th that of steel, so even with a larger size line, they will weigh less. Compared to .008 steel cable, perhaps as much as 1/4 to 1/2 the weight. The drag is going to be a little higher than .008" steel. (I haven't run any of the numbers. Too lazy.)
I think the industry is following the convention that "pounds test" means the minimum breaking strength. So if you apply a factor of safety (FOS) of two to the 30 lb line, then you never want to stress it above that number, or 15 lb. I think the highest pull test I recall for any 1/2A situation was 7 lb. So accounting for two lines gives a huge safety margin.
There was a lot of discussion about the spectra strength being "cut in half" if proper knots, use of lubricant when seizing the knots, and considering handling and abuse, and phase of the moon were not acknowledged. So I picked out the package that the fishing store had on the shelf that said "Made in America," and took my wallet to the checkout stand.
The higher rated line isn't necessarily wasted. Go read the opinions and some data on the line stretch. Since the goal here is to ensure all 1/2A addicts are capable of 500 point patterns, the issue of line stretch came up. As regards control feel, line stretch was postulated to be excessive using Spectra lines when sized to minimum strength values. Since I have yet to score higher than 8's on some maneuvers, I wasn't too worried about line response--but you can never be too careful!
Really wasn't too much more thought involved than that. And it works pretty good for 1/2A's. And especially hinges on any size plane you can fit into your garage.
The Divot
PS--The manufacturer's data for their 20 lb test line gives the diameter as .009". No reason it should not work fine.