I have a question to you: what do you think, if I send 1500 imps to the mines, what is the pay back time?
Though it wasn`t a question for me, I`d still like to answer. You also have an option to spend them on artifacts - though at this point you should study items market very carefully, there is a certain decline in items trading.
I advice against item trading at this point. The best solution would be to put those IMPs into mines, I think items system will be somehow reworked in future - it`s much safer to keep IMPs in mines at this point.