The 6g SSDs are indeed very nice, but I feel the MBA is bottlenecked by other things than the stock SSD, which is pretty fast already, especially if you have the Samsung version.
You might shave off 1-2 secs on boot and apps that take 3 secs to launch will launch in 2.5 secs, other than that I guess the biggest increase will be on synthetic benchmarks.
In a year or so when 256GB+ SSDs will drop in price I guess a little speed increase bundled with expanding your storage will be worth it, but dropping $600 on a little speed increase for a 240GB SSD is just too much, at least for me. The SSD alone is costing half of what the MBA costs