EnuoEnkidu

Ok, first up the pipe going from the exhaust to the throttle body is an emissions control device designed to reduce the hydrocarbons in the exhaust gases. Unburnt fuel in the exhaust gases is sucked back into the engine for a second cycle for more complete combustion. Blocking this pipe off increases performance by increasing the percentage of the intake charge that is oxygen (i.e. more fresh air) so more fuel can be burnt, hence more power. Second up, exhaust valves are smaller because a vacuum can only exist at pressures down to 1 bar less than standard atmospheric pressure, and vacuum draws air into the cylinder(s), whereas exhaust gases can and do exist at much higher pressures than 1 bar and therefore can be persuaded to travel through much smaller passages. As has been said before, gearing is the only way you will increase the top speed of a bike that hits the rev limiter in top gear. It is probably worth bearing in mind the variations in speeds people claim to have seen are more likely to do with tire pressures affecting the overall wheel size: an under inflated rear tire will lead to lower top speeds etc. If the ignition timing is purposefully retarded to avoid pinking, as has been claimed, you can increase the power by advancing the timing, but high quality fuel and/or an octane booster fuel additive will be required. This will probably be worth, at best, half a horsepower. A well designed de-cat exhaust will improve power, but a larger diameter exhaust will sacrifice any low down torque then engine may have for high rpm power gains, and go too big and you'll lose it completely. This is because exhaust gases have momentum, and the exhaust gases from the previous cycle help to suck the exhaust gases from the occurring cycle, meaning there is again more space for fresh, oxygen rich air. Again, worth maybe half a hp if done right. A ram air pipe or similar induction kit will undoubtedly help, although again, realistically you are looking at maybe half a horsepower, if that. Increasing the compression ratio is out of the question if you want to avoid pinking melting piston crowns, but you might get cams that can give more power but as is usually the case, low end torque and therefore drivability will suffer. People imagining gains of maybe 2 or 3 horsepower for say, an exhaust are expecting a bit much from an engine that puts out 14 ish horsepower from stock. K&N quote a 3 horsepower gain for an induction kit on a 2 litre Honda Accord, and as gains are proportional to displacement (think about it, a supercharger on a Corvette is going to add more horsepower than a supercharger will on a Ford Fiesta) you can expect maybe a 10th of that for a 125cc engine. Actual power will be hardly affected, but the responsiveness of the engine (speed with which it reacts to changes in throttle) will be improved drastically. Ultimately though, with the amount of money you spend buying mods to try and beat 100mph, you could pass your test, and PX your 125cc for a 250cc, possibly one still in warranty because it hasn't had exhaust, induction etc mods done to it...