I finally got the pickup in tonight, got it restrung, played it for a couple of hours, and I've reached my verdict... It's exactly what I wanted, and it sounds better than I expected!
The custom custom is insanely fat sounding, its dark yet articulate and has tons of mids. The bass has a lot of character, its not booming or tight but it has a vocal quality to it, thats only there on the bass and not on the rest of the sound. Chords, even distorted are distinguishable. The clean sound is quite good, a thousand times better than the stock Gibson pickup. Although its a rather dark pickup (which suits me perfectly, being in an SG) it has a lot of harmonics and absolutely sings. The highs are very very smooth.
If I had to describe it as briefly as possible it would be: thick but articulate.