1. Performance, but only if you've fully upgraded your intake, headers and midpipe. Otherwise, it's just noise. IMHO, intakes and headers have a greater effect on mileage... but, who knows, maybe those tiny shower-head tips that taxi drivers put on their cars actually help save gas...

2. Depends on the car, depends on the plug. Some cars benefit, others suffer, because high-end iridium plugs often have weird head designs. For your particular car, you'll have to research websites and forums dedicated to the car.

If the iridium plug is the same design as stock, benefits include longer spark life and more stable spark... not something you'll really notice in regular use, though.

3. K&N cone filter equals around 3-5 more horsepower when it's cold, and minus 2-3 horsepower in hot traffic... ...supposedly gives better fuel economy, too, but I haven't particularly noticed any difference after installing mine.

4. High end? You mean synthetic? Advantages are a longer interval between oil changes, but they cost so much, you're still spending more. (400 pesos for regular oil for 2 changes is still less than 1500 for synthetic for one change)

It's the other advantages that are more worth your money, smoother running, longer life, better resistance to degradation from heat and friction (which affects multigrade mineral oils badly) and slightly better economy and power (depending on the oil weight).

But with our traffic situation, it's not advisable to use long oil change intervals anyway (opinions vary)... so it's up to you. You can use synthetic to baby your engine, or you can use mineral and save money.

Me? I use synthetic when I can, but not to save money.

5. Normal temp reading, at center or just a little lower, depending on your car. As long as the needle doesn't go above midline, it's okay.

RE: Mods? Depends, what car?