Choke is a relation between bore diameter and choke diameter so the fact that the bores are less than nominal 12b is irrelevant. If it has 10 thou constriction between bore and choke, it has 10 thou constriction, irrespective of the actual bore and choke dimensions.
As SKB says, what you take out, you can't put back, so try various manufacturer/wad/shot size combinations to see if any give you the pattern you want before butchering a nice gun.
Accepted wisdom is that fibre wads reduce effective choke but research in the UK with modern powders/choke/shot has produced evidence to question this assumption. You will have to experiment with your own gun/cartridge combination.
Also don't assume things: I did when I was first shooting and changed from 6.5 to 7.5 UK shot size to improve my patterns. If fact I found I got seriously blown patterns even though the cartridges were from the same manufacturer and identical in all other respects. Basically my gun didn't like them.
In UK parlance, your chokes are tight half and loose full which is traditionally too tight for anything but trap BUT I wouldn't change a thing until you have exhausted all other possibilities.
