Hi Bianco:
I've had Wilsons since the WP6. I thought the biggest improvements overall were between WP 6 and WP 7. But even more so the WP 8 to Sasha. The Sashas are significantly better in every way, especially in a more natural midrange than the WP 8. Obviously, few have heard the Sasha 2 but based upon the design changes, it looks like the biggest difference will be in the tweeter and the crossover from the mid to the tweeter.
I have heard the Alexia and XLFs on numerous occasions and can say that the tweeter has a slightly different sound than the tweeter that you are used to. I have not yet decided if I like the new tweeter better than the old. I never really found the old tweeter lacking. May be a great time to grab a used Sasha.