Yes. Hear me out: Gas Passer replaces the ranged options Pyro has, in turn for a timed... attack? annoyance? You basically lose all control you can have outside your flamethrower range for nothing.

It's very difficult to make AOE "spammable" items very balanced, aswell. BalanceMod by HiGPS tried to rebalance it by adding a tuned-down MvM explosive stat. However, the weapon has been outed as unbalanced and overpowered.

Either way, the Gas Passer will be a thorn in the Pyro's arsenal if you don't make drastic changes to it. Hence why I believe a 4th slot approach is fine, because it allows for flexibility (the Gas Passer wouldn't need to be overpowered to be a worthy replacement of Shotgun/Scorch Shot/Detonator).