Perhaps its exchange side, perhaps not.
ADSL requires good tech on both sides. Sure, the exchange may have been upgrade, but you did say connecting to the master socket and altering your router to an ADSL2+ compatible device improved the speeds. The chances are -nothing- has changed on the exchange side, but by connecting to the master socket you have removed and crappy internal wiring you may have had the router going over, plus any interference from other cable runs it crosses and other devices it snakes past. All of which will likely cause drops, which in turn will drag your overall throughput down.