IE's Steel Axe adds +10.5 damage
All vanilla axes (including diamond and excluding wooden) add +8, with the attack speed being the changing variable instead. Generally mods seem to follow that trend, with examples being Botania, Thaumcraft and Psi, so IE's axe probably is a bit uninentionally op right now.
In pracitcal terms, Zombies, Skeletons and Creepers all only need 2 instead of 3 hits from this absolute unit (w/o enchantments). :)

Addendum:
Discord User @Nuzzleskatev#1 had this to add regarding the Steel Axe attackspeed

clickable links:
https://www.wolframalpha.com/input/?i=logarithmic+fit+%7B%7B132,+0.8%7D;+%7B251,+0.9%7D,+%7B1562,+1.0%7D%7D

https://www.wolframalpha.com/input/?i=0.0751724+Log%5B424.887+x%5D+where+x+%3D+641

https://github.com/BluSunrize/ImmersiveEngineering/blob/3422abdec77ebf2a9aa63841aa991a5783eb89ca/src/main/java/blusunrize/immersiveengineering/api/Lib.java

https://minecraft.gamepedia.com/Axe