All one has to do to find out the effects of plastic-tipped hunting projectiles is shoot one 20-rd. box of them at live game. I did that years ago with the original Nosler Ballistic tips, and never again will I shoot another plastic tipped bullet. The destruction is too traumatic, and in the event you are wanting to consume said animal, meat wastage is excessive. There is only a small bit of room for error between a rib shot and a shoulder shot, and I have seen way too many shoulders ruined by too explosive a bullet.
Deer are not hard to kill. A standard cup and core projectile will do wonders in most cases (although I HAVE seen Sierra Pro Hunters fail miserably at reasonably pedestrian velocities).
Finally, just guessing here, but I would expect muzzle velocity from a slug gun to be about 1900 and from a ML about 1400. That is 65% and about 45% of what you should expect as MV from your .243. I think the damage potential goes up significantly with speed. What happens if it is literally the buck of a lifetime and that plastic tipped bullet explodes on the shoulder without penetrating to the vitals? The .243 is minimal in my book anyway. Why up the risk? Get some Partitions, Grand Slams, or similar, and use them.
Just my honest $.02...