But it would really be cost prohibitive. You would do better just selling it and buying the top of the line current machine. Your other option is to go with a MacPro for real speed.
You can go to an A1398 Retina board possibly this one: Apple MacBook Pro "Core i7" 2.6 15" Late 2013 (IG) Specs
Identifiers: Retina Late 2013 15" - BTO/CTO - MacBookPro11,2 - A1398 - 2674
But it would really be cost prohibitive.