Eagles eye and Farenheit both got crappy upgrades. Same with a couple others.
Save those weapons for your last upgrade- as others suggested.
I think that's expecially true for fahrenheit. Eeye has a little difference in pvp with +2 damage, fahreneit with only +1 has a ridicolous upgrade.
But we have to keep in mind that most of weapons without an ability with activation cost are not so much different from their upgrades: jade staff, titan, arsenic, morning star, discord; are better if upgraded, but their key is in the ability that's always the same. Only stiletto improves his ability with upgraded damage, but it simply becomes playable (as unupraded tends to be an useless card).
Eeye isnt an exeption