I opened with Pyfa and it confirms that i am right.
425mm Prototype gauss gun and iridium with all skills level 5: Each turret deals 21.2 DPS.
425mm Railgun II and iridium with all skills level 5: Each turret deals 23.3 DPS.
So yes, it should be more damage.
And yes, Spike deals a lot more damage than Antimatter. 46 DPS versus 36 DPS per turret.
This is all ignoring the Magnetic field stabilizer, which brings the difference between each turret to 17 dps per turret if you have 3 magnetic field stabilizers equipped. This is a whopping 136 DPS difference between Tech 1 and Tech 2, for Spike and Antimatter.