The only downside with adding a reinforcement to the vehicle roof is that you potentially destroy the very good earth you created with the underside of the mount, ideally any strengthening added to the roof should be bolted/welded to the roof at a number of points to ensure that a good contact remains.
A friend of mine had an intermittent high VSWR on his screwdriver antenna, he had added a thick sheet of alloy under the roof skin and dissimilar metal corrosion eventually destroyed the electrical contact, the same will eventually happen with a saw blade as it has a high carbon content and be prone to oxidation, it will work, but if you begin to see odd VSWR readings over time, that would be my first port of call
A friend of mine had an intermittent high VSWR on his screwdriver antenna, he had added a thick sheet of alloy under the roof skin and dissimilar metal corrosion eventually destroyed the electrical contact, the same will eventually happen with a saw blade as it has a high carbon content and be prone to oxidation, it will work, but if you begin to see odd VSWR readings over time, that would be my first port of call
