None. No Difference. Ethernet CAT6 is more than capable of reaching even 1GBPS.
Changing your cable to CAT8 won't change a thing unless your Plan is more than 1GBPS, and even then, you'd need special equipment to handle speeds faster than 1GBPS.
TL;DR: None, no speed increase will occur. The speeds you'll get will still be 200-300MBPS.