You just have to learn to lead the Grenade Launcher to hit enemies. I think the only way is through practice. I like to fire three rounds at a time so I have one in reserve in case something surprises me, but others might prefer unloading all four rounds. Remember that after fired the pipes linger on the ground for a few seconds before exploding... so they're great for blocking hallways/corridors/areas. Pipes bounce around corners too, so sometimes they're a great way to lead with some damage before going in for the kill with Stickies.
I'm not so sure I agree about the Loch-N-Load being OP. You lose a lot when equipping that thing: your clip size is cut in half and pipes don't linger and cause area damage because they break on contact with the ground or walls. I think if you equip the Loch you're significantly limiting your damage output and area denial ability in favor of a few more deathmatch kills. And that completely ignores losing the ability to bounce pipes around corners, which is just so important for the Demoman.
As for the Sticky Launcher... left-click, right-click, left-click, right-click... and so on. You need to be airblasting those things a lot. Some people use it as purely a trap weapon but I prefer it as a way to deal a ton of area damage over a short period of time. I know it feels "slow" sometimes, but if you're arcing Stickies from that juicy mid-range sweet spot, especially with other classes near you soaking incoming damage, you can wipe out entire teams. Oh, and don't forget that holding the first button charges the Sticky Launcher so you can fire a single Sticky for a super-long distance. With some aim, you'll nab a few more kills and destroy a few more buildings.
Finally, and I say this as someone who plays as a Medic most of the time, you're killing your Medic when not carrying the Stock Launchers. Not physically, but he's still dying inside nevertheless, slowly and painfully.
Demo's bad at close range? Blasphemy! Use the pan and you'll be invincible.
Honestly, only use stickies for traps and reaaaally good pyros. Use pipes for everything else. Stickies are slow when you need to dish out damage really fast and get away, and it gives you carpal tunnel. Lay a stickytrap on a point and learn how to predict and use the pipe launcher for everything else.
I totally disagree with this. A Demo should be using pipes and stickies contextually. If you can aim stickies reasonably well, you can either block opponents from moving in certain directions, or kill them if they refuse to change course. And they're not that slow (though I suppose that is subjective). I actually think, if anything, a Demo should be leaning more heavily on sticky use than pipe use. But really, a Demo should be flipping between each of those weapons depending on what he is trying to do at any given time.