<== I installed a reverse P (Seymour Duncan SBP-2) and just routed whatever was necessary to fit the pickups in the reverse orientation. I did not fill in the original routs. I love how it sounds, but did not do a regular vs reverse sound comparison. There is a video on Youtube comparing the reverse vs regular P pickups in two BC Rich Eagles (I think Eagles, some kind of BC Rich). I could hear a difference, but I also think it looks cooler to have reverse P pickups. My eyes may have heard the difference.
When I add bridge pickups to my other two Precisions, I will put them in the reverse orientation, but leave the neck pickups in the standard orientation so I don't have to order or make a custom pick guard because I am cheap. SPB-3 pickups are great for the bridge position. The split between the two pickup halves is in the 60's jazz position. I think the P pickup in the bridge position is way better than a jazz pickup.
I have never routed or removed other material from beneath the pick guard area, so I can't comment about the effects of doing that.
Good luck with your mods!