Re: wiring diagram for push-pull humbuckers
A capacitor will just filter out some of the frequencies, usually some of the highs, and won't add any volume.
What's far more useful than coil cut (what you have now) is spin-a-split. It turns the tone pot into a volume pot, so you keep part of the 2nd coil on, which has more volume, mids, and lows than coil cut, but not as much of those as full humbucker. Instead of going from 2 coils to 1 coil, you can go to 1.5 coils, or 1.75 coils, or 1.25 coils, etc. There's diagrams online for it, very quick and easy to do.