As far as im aware you can get away with 1.5mm. Not sure if you can use more,i suppose you could use a de comp plate and double up(a gasket each side). Timing should be ok using a single(thick)gasket but more than that i would think you may have timing issues. Arent the zetec cam pulleys tappered? Maybe you could use them like verniers?
If the comp ratio is 14.1:1 then 1mm should lower it by 1 comp point to 13.1. It will be a little higher than 13.1 as its very rough,you would only no for sure by striping and wet testing.