I'm glad that you decided to educate yourself on the topic. Here's where you went wrong.
1) Trunks actually says "Is that so?" in the original dialogue. Viz changes it a bit. I honestly don't think anyone except Goku has gotten a sense for Cell's real power, I think the rest of the group assumed Base Cell was all he had. Goku shocks them.
2) Interesting note, it does seem Goku and Cell get a feel for each other's full power. Very interesting. With Goku it's clear, but Cell's comments certainly point to him getting a good sense of Goku's power as well. I had not known this previously, thanks for indirectly showing me this.
3) Grade 3 power certainly shouldn't be utterly useless. In a pinch, turning Grade 3 and doing a max power ki blast obviously would be very, very beneficial. It isn't suggested in the story except for Roshi doing it, but they really all should be capable of doing something like this.
4) I think Trunks knows Goku is holding back power after his conversation with Piccolo. It doesn't change Goku's placement against Cell though, only against their expectations.
5) Vegeta is clearly aware of something being wrong. If you have ever seen what Vegeta looks like when he truly feels he is the most powerful one, check out the Androids Saga. Or right after the ROSAT. He is the most arrogant motherfucker known to man. He immediately rushes to battle and he forces the action from the start. This is certainly not Cell Game Vegeta, he is putting up a front like he's the strongest, but he is being very cautious while holding onto his ego. You have to read between the lines here noobie.
6) Piccolo has to realize Goku is holding back, he has no sense on Cell's power, he only has a sense of Goku and how Goku is. He is basing everything he thinks about Cell based on Goku as a guideline, so is everyone else. That's the overall point you are missing.
7) The Cell Junior fight with Piccolo is what puts Piccolo somewhere near them as well as Cell's comments about someone other than Trunks and Vegeta possibly being on par as well. Piccolo is standing at the end of the battle, that is absolutely a feat, try hard as you may to diminish it, the Cell Junior was simply not powerful enough to put the Great Demon King Piccolo down for the count. Meanwhile, Cell Junior was powerful enough to put down freaking Full Power Super Saiyan Goku, who was fatigued, but no matter.