hi, what is the best modification one can do and easily reversible when due for inspection??thanks?

Question

1.5 steptronic cvt mivec engine-colt plus

Answer 1

1. Computer performance chip. 10 minutes to revert
2. Cat back exhaust. 1 hour to revert
3. High volume air filter. 15 minutes to revert

Answer 2

NOS (Nitrus Oxide), will cost some dough though.
You can put some legal high-perf. headers and exhaust that you could bolt and unbolt the cat easily.
Those will give you quite the performance difference that you seek.
Re-program the computer chip.
But you'll have to do some suspension work.