Your right, if I understand correctly, 340 mission discovery drops shouldn't be guaranteed from a 340 mission discovery mission. But, I have ran at last count 31 of the 340 purple missions and I've gotten 2 mission discovery drops. One was a slicing mission (cool) and the other was a Dark side, I think diplomacy, drop. Thats with my comp being at 10k affection.
Apparently the only comp that I'm going to have with +anything slicing crit is my r2d2 droid that is on my artifice. It would be nice to determine your own comps based on the crating prof you choose. The only one of my character where the comps somewhat match up is my smuggler that has cybertech, scavenging and underworld trading.