Is it an air cooled or water cooled?
I raised the CR on mine, because the head face needed a skim anyway as the engine had eaten something metal (long story).
I cant remember how much by. I was targetting increasing the CR by half a ratio, its always been my approach to do this in any tuning project. I went through the proper process of measuring the existing chamber volume using a burette, calculating the required change in chamber volume and then buretting it again and measuring the amount to be skimmed, slightly fiddly but your porting man should have done this 100 times in the past and know what he is up to.
I think the result was either 0.5mm or 1mm, i seem to remember a roundish number.
It gave a really noticeable extra shove of torque at lower revs, I didnt feel so much change at the top end but at the time i was getting into marginal fuelling and marginal air flow up there because of the fuel injection system i was using. I did notice it got close to the detonation limit which hadnt been the case before. Therefore I would suggest that on a bis, half a ratio is ok (i.e. if it was 8.6 before it can be taken to 9.1). I cant remember the standard figure off hand though. I do not think it would be good to increase by a whole ratio.
fightingtorque