For T foils to be effective they don't have to be behind the transom of the boat.
That is accepted, at the moment, as the most logical and easiest place to put them, but if they were placed as a fixture under the boat finishing at the transom or fitted as a retractable "small" dagger board (with T foils) through the hull behind the rear beam of the hull, they would be just as effective.
You are correct in saying they make the boat “feel” as if it is longer, but that is only symptomatic, the effect that they have on the handling of the boat occurs in quite a different way than it would if a couple of feet was added to the waterline length. The foils control the horizontal directional way in which the hull travels through the water in a more efficient and controlled way than the extra length and buoyancy of a lengthened hull would. To look suspiciously at the addition of T foils would be similar to arguing that dagger boards should not be allowed as they improve the performance of the boat and a smaller boat with “boards can out point a larger boat without boards so that the smaller boat can sail as if it has a couple of feet more in waterline length?