That's a big compromise. For every advantage you get, you also get a disadvantage.
The big advantage of the A8 is its firepower. Maneuverability, climb, and speed are found in other 190 models (although the a8 has decent speed).
If you're flying the a8 with just 2x 20mm, IMO you're giving up its biggest advantage (firepower), and taking all of its disadvantages (average climb, poor maneuverabilty, average top speed).
Between the 30mm and 20mm option, you have to examine the guns characteristics. I don't know which weighs more, but I'd assume its the 30mm. It's important to understand that you're losing climb rate, speed, and maneuverability when you take an of the extra guns - I don't think a lot of people realize this.
Obviously the 30mm option has more hitting power than the 4x 20mm. If you hit, that is. The mk108 has a low rate of fire, horrid muzzle velocity, large bullet drop, etc. It's not called the tatergun for nothing. This makes it useful only in dead 6 shots, from relatively close range. The mk108 also has a low ammo load (55 rounds per gun, I believe). This means you don't have a lot of firing time from it, and after its ammo is gone the gun itself is just dead weight.
In a long range shot or a high-deflection shot, due to the difference in ballistics between the mk108 and the mg151/20 you're only going to be hitting with two guns since they cannot be fired separately. That means two guns are being wasted, yet still consuming ammo. Not worth it IMO.
I take the 4x 20mms every time for a number of reasons. All four guns have the same muzzle velocity and bullet trajectory, so if you're on target he's getting a hell of a beating. If one gun is hitting, they all on target (convergence aside). 4x 20mm is plenty of firepower to bring anything in the game down. If I have a dead six shot in an a8, I generally just tap the fire button. Less than a quarter second - plenty to take down any fighter. Sure, you may have to hold on target a bit longer for buffs and other extremely tough targets, but with the a8's ammo load, you don't have to conserve ammo.
I do well in the 190a8, and never take anything but the 4x 20mm. If I want a different firepower option, I'll take a different plane.