If by "factory setup in the $300-$500" range you mean a Kala, Lanikai, etc. not set up by a retailer who specializes in ukes then you could see significant improvement in playablility (and thus progress as a player) just by getting a good setup on your present uke.
Improvement as a player mostly comes with playability and intonation of the uke rather than from the "bling factor" of a custom or a "k-brand" - if the action and intonation are both very good it really doesn't much matter what logo is on the headstock or how much the uke cost. The more expensive uke may be louder with better tone and sustain, but action and intonation are the really important factors for player improvement. Sure...there is typically a correlation in that the more you spend the more the action and intonation are likely to be good...but it's not a 1:1 kind of thing and, as someone mentioned, you run into that law of diminishing returns.
Frankly, if you are not progressing on a $300 uke that is set up properly with a decently low action and has the right strings chosen for decent intonation all the way up the neck then you are unlikely to progress any faster on a $2k custom. On the other hand, if your $300 uke is a typical example of a Kala or Lanikai straight from the factory you are likely to progress much faster on a better uke, be it the one you have after it being set up properly or a $2k custom.
Only time and practice you will improve greatly unless you are musically gifted.. play more ukes and in time you will figure more things out.. I call them aha
