Don't go with a really fast LB; his SPD is gonna increase ~33%, so if his speed starts at 70+, you'll lose potential improvement as you approach the 90 SPD cap. FYI, if you look at the player stats, there are no converts among the top sack leaders. They're usually good, but seldom great. It works out best when you lose out on quality DLs and are stuck in the middle of the draft needing a DE.