Not for Mac Minis, but you can get two port, Intel X520 series 10GbE (optical) PCI-Express cards for $85 to $110 each on ebay these days. 1310nm/LX SFP+ are really cheap, like $25 a piece.
I am biased, working in network engineering, but I really don't see a huge use case for 10GBaseT. It's almost unheard of in an ISP environment. Anywhere we need more than 1000BaseT we do everything optically.
What about connecting to storage, either an SSD NAS or a local disk array? At work we have regular old HDDs in RAID 1 and it saturates a 1 gig connection, so I'd imagine SSDs in some faster RAID would get way above that, and you don't have to redo wiring everywhere if you already have Cat6 installed.