Thanks fam. This is a really good question that I never thought about before.
I'd say that's a good possibility. The thing with Piccolo is he has no transformations, so it gets a bit tricky assuming there's a set increase for him. We've seen him go from a resting level that #20 thought had a lot of energy...to Piccolo fighting at a level that only seems possible for a Super Saiyan. When it comes to Saiyans, I think it's a bit more simple because the transformations seem to yield a consistent boost in most instances.
Back to Piccolo, I tend to look at it from this standpoint: Goku took a look at Cell up close and had a feeling that he wouldn't be enough to beat Cell. This was reaffirmed by Karin. Piccolo, though powered-up, wasn't at a level where Goku would have a reason to believe he surpassed himself. Knowing that Piccolo wouldn't come close to his full-power, he'd already have enough information to disregard Piccolo's power altogether for the Cell Games.
If I had to use numbers, maybe something like this: Pre-Rosat Piccolo 40-->Post-Rosat Piccolo 80. When he'd get serious and fight, Piccolo would probably jump to 120 or so. So while it's still guesswork, Goku's perception alone would be enough for him to reach the conclusion that Piccolo was improved, but not enough to make a difference in the tournament.
Trunks revealed that Vegeta surpassed Super Saiyan in two months, so despite not seeing it, he knew that Vegeta had the form. Once Goku acquired both forms himself, he knew that becoming a Super Saiyan and maximizing the power of the form to eliminate its weaknesses would make him much more powerful and effective in battle. This is the main reason why I believe Goku could accurately gauge Vegeta. Vegeta quickly realized that Goku and Gohan's way of utilizing Super Saiyan was more efficient for battle. Spending time sparring with the kid who went on to become the most powerful Super Saiyan also helped.