That interesting that you need to underdose....
I found with my 18 & 22g VST`s that I neded to up dose to approx 20 & 24g to get the results I wanted with the Expobar
I don`t think it matters how many grams you fit in the basket just make sure the volume is correct
The amount of coffee you use will vary between beans a roast levels
Use the five cent peice test to check your level
The VST`s are sensitive to dose and distribution, I have switched back to using my standard type baskets which I prefer .