Hyundai transmissions aren't the greatest at economy. I've got an auto Hyundai myself. DO NOT. I repeat that, DO NOT! ever engine-off coast. Best you can hope for is neutral, engine-on coasting. Unless, of course, you want to guinea pig EOC in a Hyundai and find out what happens. It'll shred the clutch packs.
V6, eh? That'll be a tough beast to break. Coast a lot. Flush/fill the transmission with fresh fluid. New plugs/wires. Coast. My 4-banger Hyundai does NOT like warm-air intake; not sure about the 6's. LOTS of coasting. Upper grille block should be no problem. Check the MAP/MAF sensor (not sure which you have). Did I mention coast a lot?
Rear window aero on my car is HORRIBLE. Kammback/roof spoiler would be a HUGE help.
Best of luck to you!