The problem with supersonic agility for dogfights is that you can't actually use supersonic agility. The faster the aircraft, the greater the g-loads imposed on the aircraft. This means that a Mach 1.5 aircraft pulling 9 Gs is reduced to about 11.5 degrees per second (this is ignoring ITR vs STR) and your opponent is now much more agile than you because of their low speed. You have to remember that a traditional dogfight is an attempt to get a firing solution on your enemy without your enemy getting a firing solution on you. To do the 180 needed to catch the opponent's six, you'd need around 10-15 seconds.