You know how you can't really notice the faster weapon switch on the RS on the soldier, the only effects taking effect during weapon switch-heavy play? And how pyros don't really utilize their faster weapon switch all the time, instead keeping one weapon for a few seconds until another is needed? The extra 15% is not a problem at all. Such small switch increases only work with weapons that have long fire times and long switch times. A Degreaser pyro has neither.
I dunno if I gave my analysis on this change yet, but here it is again if that's the case. The pyro already has two weapons that provide increased damage in specific situations, the Axetinguisher and the Flaregun. One has more raw power while the other one has more range, but both lack flexibility and versatility. The flaregun has great ranged capabilities and can deal a good bit of damage, but is very slow to use and can't deal with more than one threat at a time. The Axetinguisher is pretty much the same, exchanging range for killing your enemy before they realize you are there. Even less capable of dealing with multiple threats because you die so fast at that range.
The Reserve Shooter exchanges both extreme range and raw power for increased versatility. It is much more capable of dealing with a multitude of situations and threats a Pyro has, like scouts kiting like the little bastards they are, or rocket jumping soldiers, or anything they just airblasted. It won't do as much damage as either weapon in their optimal range but deals a decent amount in most ranges, no real weak spots.
The Pyro still has a major weakness to multiple targets with the smaller clip. Airblast can control crowds and help you escape unfortunate situations like that, but you won't have any firepower to deal with any followers.(Do note the irony of the word fire in that sentence)