Tcrafty,
Whatever you decide to do with the firewall is not as permanent as it sounds. I have changed the amount of down-thrust and right-thrust using washers between the engine mount and firewall. A little experimentation with your current Sr. will help you visualize what Bruce explained.

If you decide to build the plane with less down-thrust the main risk is it might make the plane want to gain altitude when you advance the throttle after trimming for level flight. The Kadet Sr's design dates back to the '60s and has a very efficient, flat bottomed airfoil (perfect for low and slow!) that can be very sensitive to throttle changes resulting in the pilot constantly having to re-trim the elevator to maintain level flight. It is possible that Sig increased the down-thrust setting at some point as engines became more powerful.