Ahhhh well then yes, he's be exactly between SS2 and SS3, 3 being 4x SS2.
Both forms would be rapidly draining Ki, but the SKK form would also be putting a strain on his body. Not to mention that it may be more taxing to use it with SS2, making it drain faster (as a half idea to why he doesn't/can't use it in canon)
From what I can tell, Goku was a regular SSJ when he fought Pikkon, not a SSJ2:
http://www.youtube.com/watch?v=PTepZei1Qwo
However, he logically would have still possessed power equal to a SSJ2(at the very least), given that he was roughly equal to Pikkon, and Pikkon wtfpwned Perfect Cell with 2 casual strikes:
http://www.youtube.com/watch?v=hYcEvG3rvl0
That said, when Goku utilized a base SKK, he would have become roughly 2x more powerful than a SSJ2, and roughly 2x less powerful than a SSJ3(as a SSJ3 is 4x > SSJ2.)
Given that Cell was in the process of attacking Goku before Pikkon intervened, it seems logical that he would have been powered up for a few reasons:
a.) Cell had fought Goku before.
b.) Cell is intimately familiar with a Saiyan's ability to become more powerful after sustaining massive injury.
c.) Cell [obviously] knew that Goku endured the exact same explosion/detonation that had immensely powered him up. Therefore, he would have also known that Goku had become much more powerful since their last battle.
Cell is many things, but dumb isn't one of them. That said, I do believe Pikkon owned a SSJ2-level-being with a mere 2 strikes-- and SSJ Goku was on par with Pikkon. That is uber as hell any way you slice it.
^ The last sentence hurts my head.
I believe Pikkon is strong as or more so than SSJ1 and he got the best of Cell due to speed blitz. Most of Pikkon techniques are unique to Earth. Instead of energy balls, he produced some sort of flames, which dealt massive damage to Goku. His attack reminds me of "Dragon Rage" from Pokemon, where it equally hits everyone 40 damage.
Adding more to Cell defense, he was blitz and sent to the river, which we don't know the properties in it does.
Perfect Cell, who was already SSJ2-level before Gohan killed him, was owned with a meager 2 strikes from Pikkon(physical strikes are not unique to Pikkon, btw-- they use 'em on earth too. 😛)
Anyway, based on what we know:
Pikkon=/>SSJ Goku>Perfect Cell=/>SSJ2 Gohan.
^^That might make fanboys cringe, but where the anime is concerned, it is fact.
But remember: those episodes are ultimately nothing more than non-canon filler. Hell, Pikkon isn't even a canon manga character (unfortunately.)