Some airplanes, like the Beechcraft A36 Bonanza, can realize a 400-lb MGTW increase from adding such equipment.
For example, the manufacturer of one popular STC allowing a MGTW increase for modified aircraft specifically states, "The performance listed in the basic 'Pilot Operating Handbook and FAA Approved Airplane Flight Manual' is generally applicable to this airplane with the tip tank installation at the gross weight listed in the basic manual.
Additionally, an STC may impose changes in the airplane's operating limitations, like removing a Bonanza from its Utility category certification and placing it in the Normal category, mandating a certain amount of fuel be in the tip tanks for operations above the airplane's original MGTW, lowering the speed for turbulence penetration, VA, or some combination.
Finally, in no way do we endorse operations above an aircraft's MGTW. But we've been tempted, and we know you have or will be, too.
The SR22's POH includes two tables describing the airplane's takeoff performance, one for the airplane's MGTW of 3400 pounds and the other at a reduced weight of 2900 pounds.
Now, let's add another 500 pounds to the 3400-lb MGTW, which leaves us sitting in a 3900-lb airplane.