From page 1 - stock shims: 0.1mm(brass) + 0.2mm(brass) + 0.3mm(aluminum)
So yes you need to remove if nitro 25% or less - I am running .2 with 15% nitro.
My fuel I have been running has 15% oil, 10% syn and 5% castor. Guess it depends how long you want engine to last Vs Performance
(My next bottle is 18% , have to see how it goes... )