The correlation between Tier and gpm is 0.82. The more Tier, the more gpm, and vice versa. But some values are higher than expected, some are lower. Efficiency is a difference between the value of gpm depending on Tier (blue dots) with respect to the expected value (red line).
Efficiency
Mind Breaker 110.2425
Stygian Desolator 85.7873
Pirate Hat 78.1773
Penta-Edged Sword 69.6399
Paladin Sword 66.1625
Elven Tunic 59.4525
Apex 58.5773
Ninja Gear 54.5999
Grove Bow 51.0451
Titan Sliver 49.2125
Broom Handle 38.1777
Duelist Gloves 37.5177
Ring Of Aquila 36.6851
Spark Of Courage 35.5477
Enchanted Quiver 32.1225
Vampire Fangs 31.0051
Defiant Shell 24.1725
Giant'S Ring 24.0073
Mirror Shield 22.7273
Ex Machina 20.9373
Occult Bracelet 17.6977
Vambrace 16.9451
Force Boots 15.4173
Dragon Scale 7.8251
Cloak Of Flames 7.1425
Lance Of Pursuit 7.1377
Pig Pole 5.5477
Havoc Hammer 0.0799
Pupil'S Gift -1.1949
Trickster Cloak -8.7301
Fairy'S Trinket -9.7923
Arcane Ring -12.1323
Ascetic'S Cap -14.6001
Stormcrafter -15.3301
Fallen Sky -16.2227
Seeds Of Serenity -19.1723
Faded Broach -23.7123
Trusty Shovel -25.5423
Martyr'S Plate -28.8101
Arcanist'S Armor -29.2527
Tumbler'S Toy -29.7523
Bullwhip -36.2849
Gossamer Cape -37.4149
Spell Prism -37.8701
Timeless Relic -38.5501
Ogre Seal Totem -39.6575
Philosopher'S Stone -42.3049
Ceremonial Robe -46.6975
Quickening Charm -49.2775
Dandelion Amulet -54.9775
Book Of The Dead -55.8427
Book Of Shadows -67.4127
Eye Of The Vizier -71.5749
Psychic Headband -75.4775
Seer Stone -80.4527
Telescope -95.5501