If, by Superburner, you mean a Speed Heat, those require a different control and are two wire elements instead of the three wire elements and switches on yours. The TK element you have now is quite suitable and, if it stays flat when heated, is a keeper. On Frigidaires that had raisable deep well elements, there was a channel in the side of the well where the end of the Radiantube with the connections rode up and down. Frigidaire did provide a second 8" element on ranges with that feature, but it was of lower wattage than the front 8". I think the front one was 2050 and the rear was 1700. I have never used the well kettle on mine; just kept the element up for regular cooking. When John found an old 24" Frigidaire apartment range, it had 208 volt elements because ranges used to be manufactured to cope with the reduced voltage of commercial electrical service like apartment buildings. I replaced the two front elements on my RT70 and swapped the 1700 for the 2050 watt 8" element. Now the front units are very high speed, like about 1500 watts for the 6" and maybe 2500 watts for the 8". I remember stir frying vegetables in a piece of porcelainized cast iron on the 6" element and once the pan was hot, adding the food did not slow it down for a second. "Quick cooked in dragon fire" had nothing on that experience. Before I had ever cooked on a Frigidaire, I was worried about the response time for those thick surface units, but, it you stayed close while they were on HIGH, as one should always do, and switched them to SIMMER just as stuff came to a boil, they were fine. The thing I dislike about old Radiantubes is that with decades of use, they have a tendency to warp since they are not anchored to a frame like Calrod, Chromalox and Corox elements.