I don't think of myself as "stuck" with 2.5s. I have several and they are my treasures. Personally, I only got the P3 out of curiosity, and while the boost feature is nice, the device is overkill for my style of vaping - resistances around 1.2 to 2.5 ohms, depending on the juice. I tried the P3 at 20 watts, just to say I could, but my favorite juice tasted like crap.
The P3 isn't designed for the Atlantis. From what I understand, that Atlantis really shines over 30 watts and the P3's limit is 20. If you have your heart set on an Atlantis, you need to be looking at devices with higher watt limits. Be advised that the Atlantis is too much for mouth-to-lung vaping, it's for direct lung.
Is it clouds you want or do you just want to sub-ohm? A 2.5 can fire a 0.9Ω coil and a high ratio of vg in your juice can make nice sized clouds. Not competition sized, but pretty and fluffy.
This is a 2.5 at about 1.5Ω and the juice is 70pg/30vg. They would be bigger with higher vg.