If piccolo was as strong as a ssj2 in buu saga he wouldn't be sitting around and doing nothing. He is clearly below a USSJ .
Piccolo had 0 power up after fusing with kami, even if he trained at the same rate since then (unlikely) for the next 7 peaceful years till buu saga, he still wouldn't surpass SSJ2 GOHAN.
Cell takes this with ease . at best bloodlusted picoolo will take out semi-perfect cell , who is just below ASSJ level.
Super Perfect Cell is not SSJ2 level, Super Perfect is a little over SSJ level in Cell Games. (SSJ x1.5 maybe, not SSJ2)
SSJ Gohan > Perfect Cell >= SSJ Goku (during Cell Games remember)
I see Buu Saga Piccolo > most SSJ at Cell Games.
BoG Piccolo > Perfect Cell
Super Perfect Cell > Piccolo
I haven't seen anything to put Piccolo above Zenkai boosted Perfect Cell