Gino Creglia, Jr. recently ordered the 37mm Tiffen add-on lenses which fit the Pro70's lens thread, the page linked below are some of his results.

Of course a 2.0x lens is only useful at maximum telephoto but it does mean that you get 140mm out of the Pro70 instead of the standard 70mm (and I can't see any vignetting in his sample images).

Canon PowerShot Pro70 & Tiffen add-on lens experiments