I'm sorry but I have professional qualifications and I've kicked the arses of dozens of smart alecs with certainty that they know better than the people who've done the design work. If you came to me and attempted to eat into the safety margins I've put in that would be the end of your good day. Its not the first time back-street hackers have told the world how clever they are. I work with Chinese companies on a regular basis and am certain they know what they are doing. If you've only met the back-street bakers, then more fool you.
Thats as may be, but its like anything, you can get away with things only so long. If you can show the calculations done by the OEM and the margins that you are eating into, then I'd be less critical. But I suspect that you don't have the information necessary to hack the products of others are trusting to luck.
Yes, BUT....the rest of the bike may not be up to it, especially the smallish wheels. Besides, more power through the motor or speed controller may generate more heat than they can cope with. You buy it as a package, with every specification matched, if you want something more enthusiastic on the road or track, you really don't want to start here.