I think i maxed the height of the bridge pickup. As for a heavier string, i think that really helps. I might even use a D string and tune it to "C". That's right E-A-D-D (as C).
What exactly do you mean by "high C string"?
What gauge string are you currently trying to tune to the pitch of a high C?
Or is this simply a reference to the string being mounted where a G string would usually be?
This is most unusual. Cool, if this arrangement works for you, but mechanically and physically, tuning a D string, (usually gauged .070-something) to a high C (usually gauged around .030-something) sounds like a very sketchy plan, I can't even imagine it actually working. Maybe you mean something else? Tune it
down a step from D to C maybe? Using a heavier string would definitely help support the incurred floppiness.
What are you trying to get to? It sounds like more than a volume problem.