If this is just a local event you're going to play, as long as your gun can follow the rule book for the event you're playing, your GoG Extcy will be fine.
For young guns, don't worry too much. Just have fun.
D4, you can most likely use it just fine as long as you follow the rulebook.
The same goes for D3. I don't personally own an Extcy, but I'm sure it would work fine at D3 as long as you can program it to follow the rulebook. You might get some "trash talk" because people may not think it's a good marker. To me, that gun is just like the Ion. When the Ion first came out, everyone loved it, used it to compete. Now everyone hates it, calls it a "noob" gun.
With the amount of money you would spend to compete in d4 and then move to d3 you really should really go after something a little nicer. TONS of former top tier markers you can find for less than 350. Personally I'd go with a used high end over a bnib low end every time. With the way the market is "letting someone else pay the depreciation " is a great idea. And Paintball guns arnt like cars : with a few o rings grease and Allen keys you can fix 80% of any problem that will pop up.
"There he goes. One of God's own prototypes. A high-powered mutant of some kind never even considered for mass production. Too weird to live, and too rare to die." -Hunter S. Thompson