FrogReaver
The most respectful and polite poster ever
It's really not much better than elemental weapon once you factor in the +2 to hit.
Assuming a level 11+ paladin with improved smite.
holy weapon
2d8+5+2d8 * 60% accuracy = 13.8 damage per attack.
vs
elemental weapon
2d8+5+2d4 * 70% accuracy = 13.3 damage per attack.
So 1 damage per round more for a 5th level spell than a scaled up 3rd level one.
More importantly how much damage can said paladin instantly do with that 5th level slot at level 11? Isn't it like 5d8 for the spell slot + 1d8 for improved divine smite = 6d8?
Assuming 2 attacks a round at 60% accuracy and 2d8 extra damage per hit. It will take you nearly 3 turns to overtake the paladin that just used the slot to divine smite in damage (damage now is better than damage later is always important to keep in mind).
The spell has other considerations as well. It's concentration which gives it a good chance of ending early even for a 20 AC paladin even with a decent charisma for the bonus to saving throws. You can't wait for a crit to blow it and potentially skyrocket your damage. I'd say it's comparable enough to a divine smite at the same level.