Grimiku: Crafting recipes are not on the same loot table as Legendary items, so they never compete against each other when items are dropped.
The OP is correct in that you should turn in plans since they are of no trading/selling use, ad you might find useless duplicates of the same plan if you carry it around instead of teaching it to your Smith or Jeweler. (I’ve had that happen in RoS, but just with a Royal Ruby gem design that I found 2 of in the same dungeon run.)
It’s good to know that legendary plans are not competing for space in the same drop tables with legendary items… or is it? I’ve seen lots of players talking about how they were eager to find all of the legendary plans, so then they’d start finding more legendary items. Sadly, this does not seem to be how things work, and finding, or not, legendary crafting plans has no effect on finding legendary items.