I drove around open header on my old CRX for a year, if someone can explain to me how running open header leans your motor out to the point it blows up id love to hear why.

Its a myth, its a urban legend. Its NOT true at all.

If it blows up, you were on the edge of blowing up anyway.

If its a boosted car, OPEN DOWNPIPE can actually increase boost pressure ONLY IF THERE IS A RESTRICTION PRESENT. Then in that case if you car is tuned for 10 psi and creep to 12,13,14 psi and it isnt tuned for it, then sure, it can cause detonation and damage.

If an NA car is tuned for 14.0 and for some reason it goes 14.2 open header, sure, i guess its POSSIBLE but it would be a very highly modded motor. Not a Stock Celica GTS.

100 octane wont cause your car to run lean, and it wont gain you power. It WILL destroy your factory O2 sensor though