I think you're seeing this wrong, there are 50cc french bikes that are fast as hell, but then there are 70cc french bikes that are even faster, then there are 125 cc bikes that are even faster. Its about the potential.
You cant compare Mustangs and Honda civics, they arent in the same class, weight displacement, car type... just cause you build up your 1.8 liter doesnt mean someone cant do the same exact shit to their Mustang. You can improve your volumetric efficiency but the overall hp and torque will be greater from a larger displacement motor. If you build up a 50cc motor and you do the exact same thing to a 70cc motor, the 70cc motor will have a higher hp and torque rating.
Sure you can build your 50cc bike to go fast but someone with more displacement who does exactly the same mods as you, is gonna be faster. Sure you can build a 50cc and beat a non modded 70cc bike, I dont get my kicks out of trying to compete in a different weight class...who cares if your 50cc totally modded out bike beats some lame slap in on 70cc kit, thats not really proving anything except that if you dont take the time to modify your motor you've spent the money on a kit and havent recieved all the power you could be getting. Its a fight over whos faster a 36cc goped or a 50cc moped. Go beat who you want, but dont be the civic ricer guy who spends tons of time and effort just to compete with a kid in a camaro with a calvin pissing on honda sticker on his windshield.
I wont be competing, mostly cause I dont have time, nor do I care. The best part of this is that everyone has a 50cc cylinder, this is a puch buildoff if I am correct. There isnt someone coming in with a modded 70cc bike or even a non modded 70cc cylinder trying to compete against the 50cc guys. Its more about who can attain the highest speed using only their tuning skills.
You seem pretty smart and capable. I just think spending a work week worth of work on a motor isnt very fun when I can work for 2 hours and buy kit and spend only a couple tuning, cutting, modding. I like to make things easy for myself, the best way to do that is let polini design my cylinder and let me decide how to mod it to make it fast....I see more challenge in that...making your 70cc kit faster than same exact 70cc kit your buddy uses. Or just building something for the pleasure of having it. I dont really get into the "my dick is bigger than yours" thing.