It's current health, not max, so it only outdamages bloodthirster as long as they're still above 1500 health. Bloodthirster is usually better unless you really want the slow from Cutlass to snowball lane, which is unusual because Brutalizer is usually better for snowballing lane.
Re : Ganplank builds. I play a ton of Gangplank and I've experimented with most types of builds on him, though I haven't played him since the Frozen Fist nerf (it's still probably good on him). In general you want a lane dominance item first, which means damage because you max W first in lane. Either I go dblades -> glacial shroud -> frozen fist, or I go statik shiv. Occasionally I'll rush BotRK if the enemy toplaner rushed Warmog's. In general my "core" three items are shiv/fist/warmog's, which are all extremely good on the geepster. After that lots of items are good, but they're all super situational.