Then I would suggest you rephrase your original post, because the phrase "slower than a diamond drill" does not in any way imply what you just explained. In this case, at least it does have a piurpose, though it's still not worth the expense. If you consider something that requires 16 diamonds and mines as fast as a diamond drill "OP as hell", I'm not sure what you think WOULD be a fair expense. Even the Quarry requires only 11 diamonds and the time it takes to make a 9x9 hole to bedrock is many times faster than a diamond drill, count on it.
That's the Quarry. From Buildcraft. I only run IC2, so... :p
If the 'Tunneling Drill' mined as fast as a Diamond Drill, then it would mine 9 times the amount at the same speed. I guess, considering the cost, that could be reasonable- If that's not considered overpowered.
Then again. It mines 9 times more blocks. A diamond drill takes 3 diamonds. To be 'reasonable' to mine 9 times the amount of blocks/sec, you would require 27 diamonds. Could be balanced in many ways.