Ammo GCD is 1,5 seconds, regular GCD 2,3 seconds. TP regeneration in combat is 70 per tick traited, one TP in Quick Reload equals 1,25 potency.
By pre-loading, you are going to screw yourself out of 4,5 seconds of TP regeneration compared to not pre-loading, because you are delaying the rotation and thus TP usage by that time, otherwise the overall damage should be largely identical, because you're doing the exact same thing. You trade that for a total of 750 extra potency, which is worth 600 TP. Duh, just the TP you spent.
For DPS, the biggest factor seems to be wildfire. Delaying it means a DPS loss, but so does not using it on the right abilities. A perfect wildfire (started before ammo'd clean shot into full ammo'd heat combo) at most does around 1800 damage without overheat, that's with BTE used on a non-stunned target. With 30 secs CD it's around 60 potency per second. Delaying it by 5 seconds thus around 300 potency lost. Using it right away in the pre-load instead of optimal results in 1375 damage. Using it right as you heat and load up (about 5 seconds later) would result in 1688 damage. Account for the opportunity cost of time and you end up with roughly the same results either way.
Honestly, I can see why the parser says it's a wash on the dummy - the difference should be negligible in terms of sustained DPS. And it doesn't matter if you fight players or ice there, so long as the engagement endures long enough. My own napkin math says that in a direct duel (Assuming early wildfire usage and BTE with wildfire), the pre-loader will die first, if the non-pre-loader has the damage up trait.
(First 4,5 seconds: 3750(wildfire start) vs 1500| second 4,5 seconds: 7625(wildfire boom) vs 7000 (wildfire start)| third 3 seconds: 10375 vs 11438 (wildfire boom) -> first to hit heated clean shot wins (or trait). Bonus: Fourth 4,5 seconds: 16625 vs 15438, all numbers without trait considered)
Feel free to do some simulations on your own. I personally only accounted for TP regeneration by adding a sixth ammo'd and heated shot, because the math gets finicky. I also rounded the GCD from 4,6 to 4,5 seconds for non ammo'd stuff. Latency and double weaving delay not considered either. I 'know' the flaws in my math. They shouldn't matter too much, though. And you can always win any duel by simply hitting your stun gun.
For Feast, the main reason why I would never pre-load is because you can heat up on the boxes, THEN pre-load heated shots and let TP recover. That's better than pre-loaded non-heated shots hands down, no matter what's going to come up.
On a personal note: Not pre-loading feels better. Heated shot animations are cool.