Air resistance is caused by two things.
The first is form drag. This causes turbulence, vortex shedding, stagnation points - all of which generate areas of low pressure behind the object, causing a rearwards, or decelerating force on the object.
The second is skin friction. This is a thickening of boundary layer due to general pressure gradient. As the the Reynold's number increases, the boundary leyar thickens - it gradually moves from thin (laminar flow), to transitional (damped eddies) through to unstable (vortex shedding).
Now, think about it. Your helmet is already smooth. Yet it still suffers buffet. If you covered your helmet with textile it might actually help - the reason being that you can trip the boundary layer from laminar, to turbulent, before it goes through the lossy transitional phase. This is the same principle as the "golf ball effect", and also they way that swing bowlers make a cricket ball move by polishing one side and picking the seam on the other. The polished side doesn't trip the boundary layer and actually undergoes MORE drag!
In terms of aerodynamics motorcycles are a short, and therefore relatively blunt, or "bluff" bodies. Most of the drag is form drag - you can feel this from the buffeting on your helmet. Bikes with panniers feel this as shimmy, etc, etc.
SO. It is more likely that leathers will suffer less drag than textiles because they're tight and don't billow, so they don't create extra "form" drag. The effect of them being smooth could - depending on reynolds number - actually be more damaging than them being a bit rough, but this will pale into nothing compared to the form drag effects. It seems like sticking a Hi-viz over the top of a bulky textile jacket will do little unless the jacket is actually letting air through..
The guy you quote is right to say that fitting lower bars won't help much, but that's simply because they only drop you by a couple of inches. If you laid with your arse on the pillion seat and your chin on the top yoke, it would help a lot more - look at the way GP racers tuck in. The bar position is more about allowing them the space to tuck in OR sit upright for braking, than actually dropping their body down.
As for the spark plug issue, it seems that the main reason for the change in plug rating will be the changes in carburation. Newer models will run leaner for lower emissions. Ths will make them run hotter, so need a spark plug that dissipates more heat. This leaner running and cooler running plug will also make it harder to start - probably why there are so many more people struggling to start CG125s these days.
In all honesty, if you're a skint student and want something faster, your best bet is to get friendly with someone very mechanically minded. That way you can pick up half decent bikes for not a lot of money, sort the problems out, and have a good bike at the end of it. Spending any money on making a CG125 faster is not going to be well spent - "there ain't no substitute for cubes" is an old truism. You could do SO many things from freer breathing pipes, filters, carbs etc all the way through to serious engine work. All that would happen is a decrease in reliability, increase in fuel consumption, and at a cost that would enable you to buy something faster and still have petrol money left.
arrrf.
