Aha! I found the formula in the source code on github! For hammers at least, the Stonebound mining speed bonus is:
ln( damage / 216 + 1 ) * 2 * stonebound
Based on that I've concluded that the fastest hammer is full ardite with an electric upgrade and four redstone upgrades. That comes out to:
8 (base) + 15.726 (stonebound) + 16 (redstone) = 39.726 mining speed.
How are people getting multiple upgrades of the same material? The most i can put on stuff is one full(50 pieces) redstone upgrade which only ups mining speed by 4.