Ki Multiplying
Most, if not all, cases where an attack would scale above physical stats either relates to using external energy (Spirit Bomb, Sword of Hope) or consuming life force (Tri-Beam, Final Explosion, Granolah's blast against Gas towards the end of the saga) which goes past safe Ki reserves and can kill the user. We've already brought up in the 22nd Budokai revision that Goku's Super Kamehameha multiplies power but he scales to it physically because Piccolo can tank it and harm Goku with physical strikes despite having a similar power level. That's the standard we should follow for the vast majority of techniques.
Power Scaling
For Cell's Solar Kamehameha, I don't quite understand why Vegeta and Piccolo are even mentioned when last I recall they were fodder in this fight. Not to mention Gohan was stated to only be at half power and defeated Cell with his own Super Kamehameha by going past his initial limits.
My take
The main points here are that techniques can multiply Ki and that Cell stated he accumulated power to destroy the Solar System but I have no reason to think Cell's Solar Kamehameha is any different from a Super Kamehameha (especially given it was overpowered by a Super Kamehameha) and Cell didn't charge the Kamehameha that much longer than most characters usually do from what I tell.
Okay, so multiplying Ki? If it's just a Super Kamehameha then we already know physical stats scale to it to an extent. Piccolo in the 23rd Budokai could take Goku's Super Kamehameha yet was harmed by Goku's strikes. Same thing on Namek, X20 Super Kamehameha is endured by 50% Freeza and Goku could harm Freeza. So clearly Super Kamehameha doesn't multiply power that dramatically.
What about the part where Cell 'accumulated' enough power? Well, it seems the main source for that is riddled with grammatical errors and doesn't make sense in some parts. I don't speak Spanish so I would like it if someone who does could take another look at it and clarify what exactly is being translated as 'accumulated' from the guidebook. In the Viz translation of the chapter he uses it he just says "I have enough Chi to blow away the entire Solar System!" Which can simply refer to his Super Perfect power.
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Conclusion
Despite all of that do I disagree with a downgrade? Not exactly. Ki techniques DO multiply your power. And some techniques multiply your power exponentially (via cutting life force or external energy being used). Virtually every character in DB can be 'higher' or even 'far higher' with certain techniques. What I just wonder is how big this downgrade should actually be when the only known multiplier for Super Kamehameha is around 2.3x power level and we know characters can endure it with similar base power level to the user.
The Solar Kamehameha was going to kill Gohan, so don't think that argument works
Piccolo didn't believe Gohan could do it with how low his Ki was (halved) but Goku encouraged Gohan to tap into his true power, telling him he 'hadn't used it all yet' and that he was 'afraid to destroy the Earth' and it's not like Vegeta was the only reason Gohan won,
Piccolo was shocked Gohan managed to win and pretty clearly knew Vegeta helped out. So in other words, Gohan was at half power and likely would have died to the Solar Kamehameha but Goku's encouragement let him unleash more power that he hadn't shown before out of fear. More than Piccolo or anyone else believed he had. Or even Gohan himself since he said he was 'giving it all he had' when Goku was telling him to go further.
For added clarification,
Toriyama explains how Ki works and it's stated that 'Courage' (Yuki) is an important component to it, as well as that people can give you power or energy by 'cheering you on'. Meaning Gohan was gaining power by Goku's encouragement. Hence overpowering Cell despite being at half power and everyone (but Goku) believing he was at his limits.
Gohan started that clash as much weaker than Cell and came out of it far stronger. By gaining confidence in his strength via Goku's encouragement.