As long as the iris is adjusted well, with no pinch lines on the corks, they will be fine with either italiana or portuguese corkers. If you see a crease in the synthetic corks, it means that your corker iris is out of alignment and will cause leakers with synthetics. Real corks are not usually prone to poorly aligned corkers as they bounce back into shape much faster.
I have 3 corkers, and 1 really causes leakers, while the other two do not with synthetics. I've since switched to Altec corks and have been very happy with those.