Pipes/intakes:

Yes, there are a LOT of mass produced off the shelf products and many made for the older VTX bikes will fit with some modification on the rear hanger.
There are two guys in the community that make pipes that far surpass all the cookie cutter Harley look alike pipes, they are Steve Tyre and Nate Burnell
Most cheap spike filters will fit with some mods, even the kits for the VTX's. There are a couple adapters out there that will allow you to put any Harley style filter setup on the VT1300, but require you to purchase the filter kit from Cobra or Kuryakyn etc first. However, a modded airbox and a K&N drop in will outflow all of them.

What about back pressure?
Its a 'myth'

Are there cams or pistons?
Yes, AFT makes a cam and piston package that claims a 22% increase in power, which equates to the increase that a PCV and map will gain you, but it costs a lot more and requires the motor to be opened up too.

Opening up the stock airbox is the easiest way to let more air in, especially after dropping in a K&N. Five 3/4" holes spaced 1.5" apart and then remove the center parts to make it a slot. No, riding in the rain wont be an issue.