I actually can't believe I missed it, and I've managed to get the desired effect spot on from what I intended.

Instead of cutting the floor plan out of a larger texture to reveal the floor texture, I had the floor texture on the highest Layer. I then Ctrl Clicked that to get the shape for the outside texture, expanded it X pixels until it was the distance that was desired, and feathered the selection. After that, invert the selection and mask the outside texture away, so your left with a texture expanding from the outside walls of the dungeon rooms and corridors, eventually fading out as desired, and just move that outside texture layer below the floor plan layer. After that, using the stroke on the floor plans, and not the textured layer, produces a nice stroke along the walls without giving that ugly stroke along the feathered outside edges of the texture.

Thought I'd slap this method up in case anyone is in need, as this is pretty quicker and much simpler a workflow, and the ability to stroke the inside floor without the outside texture edges is very handy.

Thank you again Midgardsormr for all the help