Perfect Cell's KMHMH was way above Perfect Cell's normal power but below SSJ2 Gohan's quickly done KMHMH, likely above SSJ2 Gohan's standard power since Gohan had to use his own KMHMH to counter that. The other one was somewhat below SSJ2 Gohan's full power KMHMH, needing SSJ2 Gohan to put focus on a full charged KMHMH when bursting out 100% of his powers.
Perfect Cell : 60
-- KMHMH : 120
Super Perfect Cell : 90
-- KMHMH : 180
SSJ2 Gohan : 100
-- quickly done KMHMH : 150
-- full power KMHMH : 200
While Perfect Cell wasn't killed by Gohan's KMHMH, whereas SPC was eradicated, I think it had a lot to do with kid Gohan not going all out in the pushing power to evaporate Cell's entire body since he wanted to make him suffer, while kid Gohan put all the pressure into its pushing power when the KMHMH overwhelmed SPC... And likely, the fact that SPC was momentarily distracted by Vegeta, created an opening that made Cell's destruction easier than it imo would have been if Cell were focused. Maybe it'd look more like Kaioken x3 Goku vs Vegeta's Galick Gun but without the Kaioken x4 part, as if that were replaced by a gradual pushing back of Vegeta and Vegeta being eradicated there... Perhaps even more quickly, I'd say.