The ShVAk has a slightly higher muzzle velocity then the mg151/20mm and a slightly higher rate of fire. It has a flatter trajectory then the mg151/20mm.
Artik didn't say he had trouble getting hits he was saying that it took many hits to get the kill.
The ShVAK has 3.9g TNT compared to 9.92g TNT in the mg151/20mm.
I think the problem here is in expecting the same results from 2 different guns. Artik said he is used to 3 x 20mm mg151 on the 109s. Well at similiar ranges the 3 x 20mm mg151 should easily cause more damage then 2 x 20mm ShVAK.
Tony Williams site confirms this
mg151/20mm = 20x82 = TW Power Rating = 16
20mm ShVAK = 20x99R = TW Power Rating = 11
http://www.quarry.nildram.co.uk/WW2guneffect.htmI suggest get in closer and fire at convergence and you will have np.