David,
The upgrade was on a set of MK 4 that I had for @ 2 years. Difference was not subtile. I really enjoyed them the way they were, but is seems like everything is clearer and more detailed. Bass is more controlled. IMHO The price after the rebate, is more than reasonable for a performance upgrade of this level.
I want to repeat that this was a very easy upgrade to perform. I had the first kit with no instructions, and it only took @ 1/2 hour total.
FWIW
The older set of Druids are in use by my in-laws, and are not able to be upgraded due to the older style tweeter, as it is a different sized cut out.