Most bows will peak at over their list poundage with the bolts cranked all the way down. As long as their strings and cables are the proper lengths, that is. So it would still be very possible to have your 70 pound bow pull 70 pounds with the tiller adjusted this way.

IF the arrow speed increases - I have seen it happen many times, but it doesn't happen every time - then yes. It is because the bow is working more efficiently.