Based on his memories of the original story, Pikkon seemed to be training on Grand Kai's Planet at this point. His strength had definitely reached at least Super Saiyan 2.
As for the power-scaling inconsistencies that so many people complained about, Pikkon had first instantly defeated Lightning Cell, yet later fought regular-state Goku to a standstill.
But once Son Goku transformed into Super Saiyan 1, Pikkon took off his ultra-heavy weighted clothes, and Son Goku was suppressed instead.
In the end, Son Goku used Super Saiyan Kaioken. In Son Gohan's own assessment, it should have been a double Kaioken.
That was because Son Goku had already reached the peak of Super Saiyan 1 at the time, with one foot in Super Saiyan 2—the so-called Super Saiyan 1.5 that everyone liked to talk about.
Only after activating that state—his cheat—did Son Goku defeat Pikkon. From this, Pikkon's strength could be estimated at around Super Saiyan 1.8 at the very least.
As for the debate over him instantly defeating Lightning Cell, there had long since been an answer: all souls sent to Hell suffered a massive decline in strength after losing their physical bodies.
Otherwise, with King Yemma's strength, there was no way he could have subdued those guys. It was obvious enough. [I can't explain it away anymore, so just fill in the blanks yourselves.]
Besides, Pikkon had attacked first at high speed—it could even be called a sneak attack—so it was hardly strange that he could instantly defeat Lightning Cell.
At Super Saiyan 1.8, Pikkon was undoubtedly stronger than Son Gohan was now, but he was not an opponent that could not be defeated.
I nearly forgot about this guy. Looks like I can meet him ahead of time.
Son Gohan seemed to have found an opponent, and he began