I think I have to agree with mkc. Suddenly after more than 100 years of using sewing machines with metal feet, this sounds like 'let's throw it out there and see if anyone buys it' kind of thing.
The upper fabric in a seam may end up slightly longer because the feed dogs pull the lower fabric and the foot pushes the upper fabric in front of the needle. The only time it really becomes a problem is on long seams like borders or sewing a backing together. Isn't that why we measure and match and pin?