I agree about the belchers, my first thought was on the guts though as they are just super iconic of the gutbusters marching line!
The big drawback in my opinion is that for the same points cost having a 12 pack of bulls procs way more benefits than four sets of leadbelchers, and their ranged damage is too unreliable for my gut instinct to say "okay, I feel like replacing some bulls with belchers is a safe and allpurpose choice" because let's face it, even if you were filling out a basic infantry line, they can't fire and advance and when they finally do get in close you may as well have picked the bulls right?
and similarly with the ironguts they just pack so much more punch for their point cost to get a 3 pack of guts over a 3 pack of bulls without taking away from your meatblock of bulls and inhibiting all of those benefits.. not to mention the 'down to the ironguts' and 'bull charge' benefits
Perhaps a good filler for this to make them a safer bet would be something like Crushing Bombardment - if any leadbelchers unit targets a unit with more than 8 models then add +2 damage to successful attacks or Cannon Fodder - every time a model is slain from an enemy unit by your leadbelchers in melee combat, roll 1d6 - if you roll a 2 or a 6 restore +1 wound
It would probably cause less anxiety about cost efficiency, especially when (realistically) to compete with a lot of armies you really need some sort of ranged unit so you don't get blown apart so hard that when you finally arrive at the melee portion of the game you haven't spent the first turn or two taking models off the board due to your enemy having run of the mill missile units
These would all be pretty fun to play test anyhow, it's a shame it might be a while before I have anyone I can play against