I'm in my iPhone now but will explain the best I can.
The head was never a major restriction, but when you think about it, everything is a restriction.
At this point though, your head was the biggest restriction, and now that it flows better, you don't need to ram (boost) more air pressure into it. It just flows better.
So, the air is flowing through the head with less resistance now, hence why you can make the same power on less boost. The air is passing through the head easier and more of it.
Anyway, at 25psi, there is even more volume of air trying to get through now. But, since the head is allowing it, something else has to be holding it up.
Lol this is always hard to explain.
Find out what max of the exhaust manifolds is rumored to be, same with the intake manifold, same with your intercooler, and find the now new weakest link.