Is car modification legal?

Strictly speaking, NO! Any form of modifications that does not conform to manufacturer's specifications are deem illegal in the eyes of law. From the simplest form of changing to low profile sports rims to changing the whole engine especially will not fit the legal category.

However, LTA does practice some flexibility. So long modifcations are not too over-the-top, your car will have no problem in passing the inspection. This allow installation of high performance cables, ignition coils and so on. Do practice some control cause TP may not be so lenient during road blocks. If they are suspicious over you car modifications, they can impound your car and slap you a fat fine.

Watch out too during body kit fitting. Any of those that protrude you car body will also be deemed illegal.

As the common Singapore saying goes: You can do anything you like but just don't get caught!

Are sports air filters effective?

Car manufacturers have taken a lot of pain to design something that is functional, offering comfort and economy of operation. By changing air tube air filter system,you may gain slight increase in power at very high engine RPM, an area where you probably use 1~2% and makes alot of noise. I doubt if you can get better fuel consumption. Unless you like to GO GO GO fast all the time otherwise  you won't be able to enjoy the benefit of the modification.
 

Does Nology Hotwires or other performance cables work? There is however a rumour that they ever cause engines to burn or operate on lean fuel. Is this true?

Nology Hotwires or other performance cables has only one function, i.e. to provide a BIG FAT spark to burn the fuel(controlled amount) that is injected into the combustion chamber. Temperature will not change much unless special fuel like methanol etc. is introduced. An experiment was done in front of me with Cheetah wires. A piece of paper was held in between the sparks. Result shows nothing happen to that piece of paper. Whoever told you about changing Plug wires will burn the engine is very shallow in his understanding of car.--sorry.

There are also saying that wires will not bring enough raw power out of the engine claimed by most manufacturers of the performance cables. To gain absolute horsepower, engine modifications such as porting has to be done. To bring up the bhp of a Suzuki Swift GTI from 100bhp to a incredble 140bhp needs at least a few days work on the engine alone. So figures on paper may not be truly believable!