Right now my set-up consists of 40lb main line which I run a sinker slider through. I then tie off to a swivel and use the same 40lb main line to create a 10 to 12 in leader which is tied off to the hook. So my question is the following:

1) If I buy 80 lb mono and use this for my leader how strong does this make my overall set-up? I guess what I'm trying to figure out is if I hook up with a big fish how much of the tension is placed on the 40 lb mainline knot which is tied to the swivel versus the knot from the swivel to the 80 lb leader. Am I any better off for running a 80 lb leader versus 40 lb?

I use 25lb mainline to 50# hook leader for abrasion resistance.

I run 30 lb mono mainline and 60lb leader mono on a lot of my set-ups.
The reel and rod do a good job of absorbing a lot of the stress.
Just back the drag off a little on the reel when need be.

I run 30lb main line and 40lb leaders. If I go any bigger and get hung up then I drag anchors and get slack in all my other rods before I can break it.

I snell my hooks and use palamor knots on everything else. Let the reel's drag fight the fish

25 lb mono with 50lb leaders here, I also snell my hooks and polomar knot everything else, for me the 25lb mono gives me good smooth long casts and is strong enough to hold those big fish while using the drag properly after the hook set, I use circle hooks so I tighten down my drag for a good hook set and then back off the drag when needed. I've experimented with heavy main lines and i didn't get the smooth long casts that I preferred and on the lake that I fish there is not a great deal of timber such as Twok or Ray Roberts so I went back to 25lb, it works for me.

Bank fishing I run 30 main 20 leader. Still catch large blues and flatties and get my weight and swivel back 80% of the time.

To answer your question, your set-up is only as strong as the weakest point. The only advantage to using a heavier (e.g. 80 lb.) leader is the abrasion resistance you'll gain. When the line breaks, it'll most likely be where your 40 lb. is tied to the swivel (unless you have a weak point somewhere in the mainline).

You don't need 40 as a mainline. Leader material is for abrasion more than anything. Sometimes, in rivers with current I ditch the leader all together and let the sinker/bead slide to the hook. It's all about knots man. Learn to tie a Snell and a Palomar knot and you're good.

25# main line, Palomar to swivel, Snell the Circle hook with 50# leader, Palomar leader to swivel.

I would suggest using a longer leader. I use about a 3-4 leader with a cork about a foot above the hook so the bait is elevated a few feet off the bottom. But that's only when I'm fishing anything deeper than 5 ft.