Goku knew Cell had reserves, that's actually why he quit. Cell was only suggested to be a bit stronger than Goku, then at the end of the fight, Goku realizes Cell has much more, to which Cell lets on he does as well.
Cell's power up is fucking massive. Look at Goku's face right there, he's paralyzed by the power. He's even forgot about Gohan's power for a second. Gohan is still much stronger, but it shows just how powerful Cell's power up should be.
Vic I more or less agree with you. In a perfect world, a transformation grants you the ability to tank the previous transformations highest amplified attack. This should be the case, in a perfect world. In reality, it may be different.
I'm glad you caught yourself about Cell's ki sensing. When he's overpowered, he appears to completely turn his ki sensing off. Semi Cell vs USSJ Vegeta and Cell vs SSJ2 Gohan are big cases of how terrible his ki sensing is at times.