The tighter bore will increase compression behind the bb as it leaves the barrel. A longer length barrel than the guns standard will actually lower your FPS and you will need a bore-up kit depending. Not sure what that guy means by seals but I'm assuming he's talking about the bucking. IMO get a 6.03 and a good bucking (KWA 2G, Systema, MadBull Shark) for every gun.
Yeah i havent looked at many tightbores and my old answer kind of applies to OEM models, madbull makes one thats 6.03mm and will increase your fps by 5-10% and will give you alot better range and accuracy