I haven't notice lead fouling in any of my shotgun barrels, although I really never clean them either as I shoot them quite often. Don 't get me wrong, I'm not saying it can't happen, Never say Never.
Out of the three slugguns I've owned, one of which was a 30" full choke until I could afford a used slug barrel for it, each gun seemed to like it's own brand of slug and or shot by brand and size. you will definitely need to pattern you gun either way.
With the slugs and a bead site you are limiting yourself on the distance you will be accurate. although a lot of deer have succumb to just the setup you have described.
Now the Mod. choke. The only thing a full or extra full will gets you is a bird further out. you should be able to nock the shorts off of a turkey out to 25 maybe 30 yards. You just need to call the bird in a little closer. Again, pattern the gun to see what it likes. Use turkey target to see how many pellets you get in the head and neck area.
I am truly thinking of going back to a smooth bore and foster style slugs as I am tried of paying 12 to 17 dollars for a box of shell. I will use iron sites or a scope though.