For example say you're fighting another pyra and let's compare regular to incendiary (greased is 15% higher dps). It takes 68 regular gat shots to break pyra armor while being repaired with mallet, although in reality a good engi will spanner before malleting and add a bit of time (but still 1 clip potential). During the 9 second cooldown of mallet you can shoot 75 shots with regular or 54 shots with incendiary, with the extra 21 shots doing an additional 278 dmg (mallet is 250). If the engi starts off by chemming (although it's usually already chemmed) that gives incendiary an extra 5 seconds of damage: 29 shots or 384 dmg. In 5 seconds regular shoots 41 shots and 552 dmg.
In every situation with a competent engi regular will outperform incendiary. Greased is 15% higher dps than regular and shoots a clip 25% faster that deals 96% the dpc of regular. In order for incendiary to break armor faster than regular the repairing engi has to screw up. The problem is that the dps of incendiary is too low to be viable in gat. We are trying to convince Muse to balance it.
Lesmok gat isn't very effective due to the spread at long range and the -30% clip. If you're planning on using lesmok gat it's better to have a hades which deals 90% of gat dps. Otherwise, it's better to have a buff engi with lesmok ammo which does more damage at all ranges. It does more damage at lesmok range, more damage in regular range, and more damage than greased!