Sure you can shoot targets at 300 yards with your lever gun, It's all about expectations. No you're not gonna shoot small groups and and a high power shooter will scoff at the size of the targets you will hit/use but if you can see it and the bullets can get there you can shoot at it. And you're .45 will definitely get there and beyond. Many many shooters use the lowly .22lr out to 300 yards and beyond, why would you not think the .45 will get you there.
It doesn't take +P ammo. You probably don't even want to use it. Cowboy silhouette shooters use nothing more than cast cowboy type loads. The benefits you gain by the flatter trajectory will more than likely be offset by many negatives.
I have heard it said one should never expect such accuracy from a handgun. I know "Hickok" on Youtube has hit his target with a Glock .23 40 caliber at 230 yards. Should that be considered as very unusual? Or do you think we should be shooting much more accurately than we do?
Most shooters that say things aren't feasible only do so cause they can't do it (not all things, there is plenty of BS floating around too). Depending on the size of the target (that expectation thing again) there is no reason a handgun shooter can't hit something at 200 yards. Heck, my .357 will out shoot some rifles I've seen at 100 yards and so it will at 200 yards too. Some shooters only think of handguns as close in, 50 feet or under, guns and never shoot anything farther. Still others think that open sighted handguns are only good out 50 yards. Both camps are 100% wrong. To be frank, if you can't hit a basketball at 100 yards more times than not your equipment is junk or you're a crappy handgun shooter. 200 yards just takes even more practice.