Lapping with sandpaper DOES change the pop pressure minutely. I've lapped existing shims AND the injector body to tweak pop pressure ~25PSI. If you're patient, you can get all of your injectors popping +/-25PSI.

Regarding the slow stroke causing the "pee", get real. The injector should positively open and close when it reaches and drops below the pop pressure. A slow stroke should have the injector "chatter". If it's peeing or squirt-gunning, it's hanging up or failing to seat fully.