The thing to remember is that you always get a Skill and Attribute roll in addition to the Ammo die. So a character with As or Bs in both have a good chance of hitting before rolling Ammo dice. I have done one play test with a friend who created a high skill operator and they hit a lot and usually only used a single ammo dice as a result.
The other bit is that the custom dice have the target emblem on them that increases bullet damage. We weren't using that as I am unsure of what numbers have the symbols, but a high Attribute, high Skill character is going to dish out a lot of damage, suppression and crits while using far less ammo than a low Attribute low Skill character.
How things will pan out long-term is hard to judge as the Virus makes getting together to game hard.