1. it's to expend exhaust gases faster, hence producing more power. depending on the type of muffler you use (freeflow or chambered), increase in the powerband will vary.

2. longer intervals of changing

3. it's to make your intake system suck in more air to fuel bigger combustion, producing more power.

4. same as #2. some say it affects/improves their fuel consumption

5. just below the mid level of the gauge should be just fine.

for spark plugs, stick with stock specs. you can try going fully synth oil also. among your stated options, i think these 2 would suit you best for your daily driving chores.

CMIIW nalang guys.

HTH