with this upgrade the new excavation efficienty is 14.1 (normally 12.8)
if i up tier 2, the excavation efficienty will be higher than the modified exacavator (15.2) and the decay will be substantially the same (0.5 pec/clic)
if i buy mod exacavator, i must pay 9kped and sell my adj ... then this solution is more economical