As Redplated suggested. Pull the tone capacitors first and see if that brightens things up to where you want it. If not then continue your search for a new pickup.
Seymour Duncan Whole Lotta humbucker is a rough cast A-5 and is fairly higher output for a PAFish pickup. The other Duncan PAFish with more output at 9.75-10K ohms is the Saturday Night Special(rough cast Alnico IV) that LPFreak highly recommends, I have not tried one yet but it's on my list but I like the WLH so much I never get around to snagging a SNS.
I also have a Steckel bridge, the Steckel is bright with clear detail but they are low output designed that way on purpose, they really shine with a higher gain amp IMHO.
I almost forgot, if you want something like the SH-5 Custom but with less mid emphasis, a bit brighter and balanced but with similar higher output then check out a Seymour Duncan SH-6N(rough cast Ceramic magnet) which measure about 12.5-13K. This used to be called a Seymourizer II prior to the Duncan Custom model, Seymour then made this wind the neck pickup for the Distortion pickup sets in the late 80's.