How do YOU determine if your cornering speed is improving? It's not a simple question ... since setting up for the next section, etc is in play. How often have you actually timed your speed thru corners and onto the straightaway or next corner?
Get a GPS logging laptimer. Then you will know. What I typically do and teach is to use the audio cues while on track. Your visual perception of speed is VERY sensitive. Going through a corner at 1mph faster, you'll likely think it's impossible and you'll die based on how your mind processes the visual scenario. You also get acclimated as your pace increases and to speed or lack of speed as you lose pace. Know what gear you're going into the corner and listen to the engine RPM's. Keep your pace up a little to keep the RPMs up and hear for the right sound increase of the engine and hold it there. Keep repeating until you start to feel the slide. Then clean pants out and look for the next challenge to fix.
It is all about where you are when you are hitting your next shift - the shift point explains everything you did or did not do correct in the corner and on the brakes before the corner. As Nicky Haden once said, "I lap until I start pushing the front tire and that is as fast as conditions will allow me to go that day."