Two suggestions:
1. Give the 5* Tortodrone guns either more crystals per charge attack (two or three), or bigger crystals per charge attack (1.5x to 2.5x as big, somewhere in that range). Right now the 3*-5* lines are almost identical, so there's very little reason to actually upgrade beyond 4* unless you're one of those super rich peeps with lots of Radiants lying around.
2. Give the 5* Tortodrone guns a small "vortex" effect on their regular attacks. If you listen to the punch it sort of sounds like it's sucking things in before shooting out the crystal. Give the 5* Tortodrone gun an effect like the Graviton/Electron vortex in a small radius in front of the gun, so the punch is more likely to hit things. It wouldn't be super big and makes enemies fly around, just a small effect to make it a bit more accurate, that only effects things directly in front of the gun. It would barely have to be noticeable.
Yea, I was actually a bit disappointed that I upgraded the my gun and discovered that it was literally the same but less damaging unless I got radiants. The gun doesn't need a tremendous buff, but some kind of progression would make the line a bit nicer as it upgrades. Like watching your alchemer grow from being a puny 2* gun that shoots meh balls that dissipate on the walls into the monster that is every driver. Something like that.