hard to understand all the stats i know...
the "poison damage" on weapons does not add damage like a spell poison would. it only does extra damage if that is the lowest base resist of your opponent. However, the major pvp special move to a katana is armor ignore, so the poison damage wouldnt really come into play unless on a basic hit, which, without damage increase would not be much.
in regards to the brightblade, the only splintering katanas worth more than a brightlade are generally: ones with higher than 20 splinter, ones with hit lower defense, ones with hit spell. And damage increase is pretty much required on armor ignore / heavy hitter weapons or the suit / template is just too much of a pain to build. Not so much on weapons you only use for specials like bleed or poison or disarm.