I'm not saying urdoinitwrong, I don't know the advanced math, especially on a DK...just offering my observation of your position; given how much HS is in a usual rotation, it seems like a huge dps loss to drop 20% of each HS off because of the diseases (and if you use a
Glyph of Disease - Item - World of Warcraft it's only one blood rune after the initial application for the duration of the fight anyhow).
Not every tier 4p is worth gunning for, especially in the face of other pieces that may be better budgeted, so you're better off looking to EJ or some deeper math heads for that advice.
Should definitely check those talents though if you're going to stick to your guns, it's kind of contradictory and a waste of points/glyphs as is :X