Gero's absorption capabilities seem to just recharge his batteries as he's not running on the same mechanism 17 and 18 are.
Imperfect Cell seemingly grows stronger each time he absorbs enough humans, getting to a new threshold of power.
Android 19 or Android 20 don't ever cross from one power boundary (Imperfect Cell arguably being around SSJ Vegeta level) to another (Android 16 level). And, depending on how strong you believe 16 is the gap could be monumental.
Furthermore, Imperfect Cell never stopped absorbing people because of diminishing returns. Instead, it was because he wasn't going to miss his opportunity to absorb the twins. They don't have a power signature so finding them was incredibly difficult for him.
The idea of non-diminishing returns would be another concept you can throw into the mix.
There's no hard and fast rule with this aspect of Cell, so I'm treating it as more of an idea-based discussion. Cell himself was surprised about what his post-perfect self was capable of, since he didn't expect he could come back stronger.
There's also the idea of Cell getting stronger through traditional training; benefiting from Saiyan and Frieza cells to bolster development.
If Cell didn't have to worry about timing, do you think a flourishing Imperfect Cell (taking advantage of all strength-gaining resources before getting to 17/18) that scales beyond his canon perfect self would still make proportional gains from 17/18 absorption?
(btw: my mention of Gero was about what he and Cell envisioned about Cell's cyborg absorption (i.e. absorbing the twins to reach untouchable power)