We all know air + gas makes combustion, however, heat also affects the combustion process. As does altitude since air is thinner in higher altitudes. There are several factors that can affect how hot a bike gets, but I'll focus on just one here, the air/fuel ratio. Otherwise known as lean or rich. Lean means not enough fuel for the amount of air in the combustion chamber. Rich means there's too much fuel for the air to burn completely. A lean mixture leads to higher temperatures in an engine which is suicide.
Hyosungs are really lean due to emmisions regulations. So, that means that hyosungs have TOO much air and not enough fuel. Jetting is the best way to bring down temperatures, increase power, increase engine longevity, and cut down exhaust popping. Putting on an aftermarket air filter or exhaust without increasing the amount of fuel to me is just counter-intuitive, even harmful.
For those that live around sea level; If you don't want to upgrade your intake or exhaust I still recommend getting slightly larger jets. (135s should work) Due to the thicker air, this exacerbates the lean problem. Sure you will get a hardly noticeable increase in power, but it's better for the engine in the long run.
But let's be honest, you want more power. So you wanna know what size jets to get. Sorry to say, there is no easy answer. There are several factors such as altitude, modifications, some say fuel economy, but that means you're not paying attention to the air/fuel ratio. Your best bet is to have someone dynotune or use an exhaust air/fuel meter using different jet sizes. Or if you wanna wing it you could add 5 for every mod you have done, (exhaust, air filter, desnorkel) don't include ais mod. then subtract 1 for every thousand feet over sea level...This is just a idea, not a scientifically proven fact. So if you have an exhaust and air filter and live at 2000 feet that's 10-2=8 add that to the stock numbers of 130, and you get 138, however, 137.5 and 140 work just fine. 22.5 is a good pilot jet size for almost all factors.
Of course learn how to check your sparkplugs. If your engine feels boggy, you might have too large of jets, could be lack of exhaust backpressure if only happens at low rpms. If you feel like you are running out of steam at higher 3rd of rpms you are probably running lean. Experimenting doesn't hurt anything, have fun, it's actually really easy to do once you know the mechanical function of the carbs parts. Lastly, I recommend cleaning the carb every year 1/2 two years, even the smallest particle can clog a needle leading to lean conditions. I also recommend header wraps to lower engine temps they also increase exhaust flow.