Im running a SOHC Greddy turbo kit with a test pipe. Since I didnt make a second O2 hole for the sensor, Im getting the CEL all the time.

Question, should I just leave it like that and nothing would go wrong if I do oil change rountinely and use only 91+ octane? Or would the CEL affect performance and damage my motor/transmission?

 

Nah it woln't damage anything except make your car run like ****. There are actually things you can do to fool your secondary O2 sensor "see" perfect A/F ratios.
I'll try and Find a link for you.

 

if you always have a cel, you will never know if you get another one..if that makes sence.

or you can buy a O2 simulator.

 

Ahh fuggit I can't find you a direct link, but yeah you can fix it with an O2 simulator without too much effort.
Good luck