fwiw to ya, and in my personal opinion only ...
i spent a LOT of time messing with carbons and have found that what's consider weak spined (500's) works best for me, even with a huge up front loading. this may or may not work for everyone, but it works perfectly for me and my bows. i found the stiffer 340 and 400 shafts were harder for me to tune.
i start off with an ics 500 shaft cut to 29-1/4" bop, 100gr brass insert, 125gr glue in point (bh, judo, fp, whatever) and a 125gr steel adapter, 4" feather 4-fletch. total arrow weight is 585gr, 350gr up front, 29% foc. i shoot this arrow out of 52-55# mild r/d longbows cut 1/8" past centershot. for lighter holding weight bows (45-48#) i drop the front end weight down to 300grs.
as always, ymmv.