Is it possible to make a 17x mega rich plasma planet? I'm wondering whether i should use my Phasic Chain-Ionizer once my planet is 14x mr or 15 mr?

Yes you can do a 17x Plasma. In fact, if you were to use Kronyn Data Cubes on it, you would be able to push it to 23x.

every arti that changes the TYPE of a planet can go above the cap of 15x mega/mega massive I believe, so it will always benefit you if you pump it up further

the "exception" is hyper terratransfromer and dyson sphere, while the latter can also go above max, because they are change-to-constant, it doesn't help you to pump them up

and far as i know, rift sphere don't give a beep about size~

other than that, if it change planet type, it would help u to max the planet first~ thou the cost might or might not justify the gain, that's up to you.

Ok. ty guys