Matrix TSD M700 Long Barrel Airsoft Sniper Rifle Help?

I want to buy the airsoft sniper riffle above and it says it goes 430-450 FPS with .20 gram bbs, but i want to know is how much slower it will go if i use .23, .25, .28 airsoft elite bbs with it. cause i want to have more accuracy but i don't know if it will slow it down to like 370 FPS or something like that. i don't want to buy a chronograph to find out but i might have to.