GT has it down. The point of leadbelchers is to weaken units up so you can make sure to win combat. Also, remember: behind the gun is an ogre. You're basically paying 13 points more for D6 shots a round, and you sacrifice the ironfists. It's a great trade in.
Also, with leadbelchers, it's not easy to always look at averages, as the range of shots they can do is very high. With 6 leadbelchers, you can do anywhere from 6-36 shots, although usually I end up getting in the 17-24 range with each unit.
In my standard army I run 3 units of leadbelchers, 2 units of 6, 1 unit of 4 in a 2x2.
Some tips from experience:
-Unless it's ABSOLUTELY crucial, don't attempt a swift reform unless your within bsb, and to be safe, general, because if you fail, you just lost all of your shots.
-Don't be afraid to charge into a unit that they are likely to win against. It's 3 S5 impact hits, 18 attacks, and 3 stomps varying, so it's not to be messed with so easily.
-Shoot at units that
1) You're about to get in combat with,
2) That they're effective against, and
3) Single models. Usually they'll be protected, but every wound counts, right?
-If you're about to be charged by a unit that they can't win against, make sure they're all within 12 inches of the enemy.
-Finally, a trick- if you're about to get flanked charged by a unit, leadbelchers are great for making sure that they don't have 2 ranks of 5 or more models, so then you still get your rank bonuses.
Overall, a extremely points effective and great unit that I would recommend.