Yeah but Piccolo was fatigued from fighting an artifical human who had equal power to him and limitless, so it stands to reason that Piccolo likely couldn't put his full power into the technique.
Fatigue causing characters to not be able to draw out their full power is usually either verbally or visually shown if so, such as Goku's weakened Kamehamehas at points in Part 1. Not only that, but Piccolo's shock at Cell tanking it certainly isn't that of someone who was both aware of Cell having greatly powered up and being aware of their own condition.
Piccolo also said earlier something along the lines that Gingertown Cell's KHH was comparable to how Goku's used to be and that was more than double Goku's power level according to Raditz's scouter, and judging by Cell being able to catch Piccolo off-guard and damage his arm, as well as the implication that Cell even then was stronger than the Super Saiyans, I don't think Piccolo was over twice as strong as Cell.
Whilst I'd agree that the Kamehameha certainly wasn't a 2x+ amp in the Cell Arc (with battle powers and amplification being purely plot dependent), it isn't necessarily debunked when Piccolo still felt the need to dodge the Kamehameha rather than attempt to counter it.
Well, Future Trunks after two trips in the ROSAT still needed SSJ to stomp 17 and 18, and Trunks was at least over 50% of CG Goku. Base Goten and Trunks still seemed weaker than 18 even taking into account the Mighty Mask suit, especially in the anime where she knocked them on their ass several times.
Goten and Trunks are significantly stronger than Future Trunks though (especially considering what the Daizenshuu has to say on their power compared to Gohan), plus Trunks was going into the fight 2 V 1 and was shown 3 years later capable of catching Cell off-guard and sending him hundreds of meters back despite Cell at least being somewhat aware of what his power was in the past. Even if we were to say the kids were weaker than #18, it wouldn't be by much with the Daizenshuu backing up what we're shown of Trunks fighting on par with #18 in a restrictive costume and the anime can't be used as a counter with it containing contradicting material for the fight such as #18 momentarily trading blows with their SSJ selves.