To actually use a 2,5g port you need much more than just a cable. It provides no benefit for internet use, you don't see more than 1gbit offered and even then most servers have artificial limits on them to prevent people with fast connection from over utilizing the server.
It all depends on what you plan to do with the fast connection. One example would be if you have a NAS and needed to copy data quickly. The NAS would also need a 2.5g or faster port and you would need a switch/router with multiple 2.5g ports. Even then this is not enough. Both your PC and the NAS need to have disk systems that can exceed 1gbit. Normal dumb hard drives can bottleneck you.
Unless you actually know what you are going to use it for...