I've always loaded either the 2x30mm or the 4x20mm, never just the 2x20mm.
Yes, the plane will be slightly heavier with the extra two cannons and ammo for them, but the 190A8 isn't exactly what you could call agile anyway- so my opinion is why halve your firepower for nothing?
With or without the 2 extra cannons, just about any other plane in the set is going to outmanuever you, so why chop your snapshot firepower in half? That is how I see it anyway.