We clearly see Piccolo powering up to what is suggested to be his full power, so no.
Also like GreatSaiyaman said, Weighted Piccolo was already stated to rival 17 and 18.
Pretty sure Piccolo mastered his training to the point where his Weights don't really affect his strength like with Goku after training with Kaio-sama.
It's likely his regular SSJ form wasn't quite strong enough to beat Cell. Vegeta transformed into ASSJ just to show off, but Trunks transformed as he decided to handle matters himself.
Also because the entire stadium was destroyed. The 24th was revived long after the 23rd Tenkaichi Budokai, which is why it became so commercialized and the level of competition declined so much.
There are too many to name. This is a pretty egregious example:
Goku was always benevolent even in the original manga, but FUNimation downplayed his less desirable traits because they thought a hero who happens to love his job and is a country dwelling NEET wouldn't be appropriate for the...
They're not morons so they definitely took a page out of Goku's book and mastered regular SSJ, but they lacked the benefit of newbie gains like Goku and Gohan so they were still far weaker after their 2nd trip to the RoSaT.
Cell Jrs were said to be similar to Perfect Cell in power yet Vegeta...
Trunks needed SSJ to deal with Cell and even then, it at least required some effort for Trunks to kill him off while he wiped out Future 17 and 18 each with one blast.