Should make a slight difference but I run a dwell of 22 on my PM7 which is also 18ms stock.. not to burst your bubble
Are you sure the bolt is passing the flick test?!
Try taking apart the reg if things don't improve, check the reg seat and go ahead and replace the piston oring, it is hard to tell if the orings are no good just by looking at them. Also go ahead and replace 009 tail orings on the bolt. My PM7 manual suggests to do that if efficiency is lacking, it prevents an excess puff of air coming out the back cap. There should be a tiny bit that comes out when you shoot but shouldn't be too much, probably hard to tell by trail though.
If no improvement then replace every single oring I guess... ?