If I may make a suggestion, you could also order a longer Toslink cable from monoprice.com for quite a bit less. They have a
25-foot cable for about $8.
I don't think that you'd notice any difference in sound. Perhaps people recommend coax for longer runs due to the cable being more robust? The link that I provided to monoprice is for a cable that gets good reviews and looks pretty robust to me.
I've recommended those adapters as possibilies for some people's systems. They get mixed reviews, though. I've never used one, but success and quality seems hit or miss with those.