Gunslingers are a solid Tier 4 to me, targeting touch AC means they barely ever miss and they get Dex to damage automatically, something that I don't think archers can easily get. I don't think it really weakens the class that a lot of builds only take 5 levels, just because their class features are front loaded doesn't mean they aren't effective at any level. There's just a lot of good feats they can take, so switching to Fighter can be a good idea.

I don't have a tonne of experience with Swashbucklers, basically no one in PFS here plays them. I made sort of a swashbuckler just before the class was announced because I watched Princess Bride, just played an Ustalavic Duelist Fighter. After the class came out I compared my character's numbers and found that I was at least as strong as a Swashbuckler, so i just took one level of Inspired Blade for the parry and riposte. They're okay by themself, nowhere near as effective as a Fighter or Gunslinger in combat in only slightly more useful out of combat. I'll say Tier 4.5 for them.

Never played a Shifter and don't know anyone that has, because why would anyone do that, and my Slayer got killed by a run of incredibly bad luck in his first session, so I won't vote on those. Not the class's fault I got blinding sickness, poisoned, couldn't make a single save, then got critted by a goblin. Guess I just wasn't meant to play a Slayer.