many people have reported on lower boost than expected. i've read that some people have enlarged the restrictor ring hole a bit at a time to increase the boost limit gradually, rather than trying it without the restrictor ring and risk the overboost. have you also got a fuel cut defencer in case of overboost, just as a precaution if you do decide to just remove the restrictor ring?
a boost controller would be a more expensive way of increasing the boost really, if it's just the restrictor ring that's a little small, but you would be able to adjust the boost whenever you like