I'm glad you noticed this, because i've never cared about welding doors that much to pay attention how much a charge of welder gets me.Not true. Just made an experiment on westlondon (suicidal, non-support class).
Police station gate: goes up to 40% on single welder charge during waves 1-5, then goes only to 20% in latter ones. Never destroyed.
Church doors: always get to 30% on single charge, no matter if they were or weren't destroyed (that includes destroying by grenades and zeds alike).
Seems like every door has personal settings changed according to difficulty of the game and number of current wave.
if a door needs welded, harass the nearest support until he gets his arse on it. =D