They aren't transformations, per se(though the power-weighted version could certainly be seen as such, given the extreme physical change) -- they're essentially just various levels of Perfect Cell's power.
First and foremost, Cell has the potential of Goku/Vegeta AND Freeza -- and we've seen the tremendous leaps they can make in very short periods of time ala DBS. He's also got Freeza's ability to *survive* damn near ANY injury, and Piccolo's ability to *heal* damn near ANY injury. On top of that, we know Cell's body has the innate ability of 'keeping' power after sampling it(that's why he still grew back into his 'Perfect' form, even without #18) -- not to mention the Saiyan trait of receiving zenkais from battle/injury, and becoming more and more powerful as the fight progresses... So knowing what we know now, just imagine what Cell could accomplish with the same 4 months of training as Freeza!? F*cking insane...
The only other character in the franchise with *that* sort of latent potential is Pure Boo, imo. Think about it: Pure Boo was already SSJ3 Goku-level, and I highly doubt that he had ever formally trained a day in his life -- all he did for eons was go through random periods of destruction and hibernation, until Bibidi found a way seal him in the 'Boo-Ball'. Thus, any increases in power that Pure Boo gained over the years were from either a.) his ability to adapt/evolve to his opponents on the fly, or b.) via absorption... So just imagine if he were to buckle down and actually train for a while..? I believe his gains would blow those of even Freeza out of the water.
And on that note, I'd bet that's why Oob will be so special in the future. He is the [good hearted] reincarnation of Pure Boo, after all, so he should have Boo's level of latent potential. Even factoring in the events of DBS thus far, it makes sense why EoZ Goku would be so excited at the thought of training Oob -- the potential there is staggering, tbh.
We did have Fat Buu train a bit prior to the tournament where he quickly lost weight and became “Fit Buu” for a short time, and gave Goku a run for his money in a little spar (ringed him out), but of course with how Toei likes to treat Buu, they had him go in a deep sleep again at the last minute so he wouldn’t be part of the main action. Sure, Frieza coming in was a neat tradeoff and all, but doesn’t really excuse the repeated neglect for Buu, which one could argue is even worse than how Tien got shafted.