Quality = everything. Unfortunately a lot of mods don't go consistently with quality
So true. I laugh my ass off at people who buy knockoff parts then complain when they explode within month.
90% of the upgrades I have on my car are actual OEM Subaru parts off different cars. The 10% true aftermarket parts are from top tier manufacturers (who typically supply OEM parts to car manufacturers).
Tuning is HUGE too. The quickest way to grenade an engine is a flaw in the tuning. If you try to do it yourself without fully knowing what you're doing or have a budget tuner try to do it, you're begging for problems. My car has been tuned and retuned by one of the most highly recommended Subaru tuners in the states. I wouldn't trust my car anywhere else.
Oh and all the wrench work is done by ME. That way I KNOW it's been done right.