I highly recommend this podcast for you:
My issue with any sort of narcotics is that it makes you into a person with less control over yourself.
The psychonauts I know have way, way more control over themselves then just about anyone else I know. The ability to handle your brain radically changing normal function, and not only handle it but derive something of value, takes incredible strength of self and is not to be belittled.
but I feel like using drugs will rob me of the reasoning skills and the willpower I so treasure
Then don't. No one is making you. But if you aren't willing to take the risk you probably shouldn't pass judgement on those who do, because it's something you don't understand. Most worthwhile experiences in life come with risks, its your call if those risks are worth it for you.
Setting an arbitrary barrier on what you allow yourself to experience seems to me to limit personal growth, not advance it. You are going to change. You're reasoning and willpower are subjected to a million external influences anyway, hunger, tiredness, how much sunshine you've gotten. Who you are will be shaped by what you experience, like it or not. The only choice you have is what those experiences will be.