My understanding from the folks at ML was that the upgrade from Ascent to Ascent i had more to do with esthetics than performance. This notion is supported by the fact that the specs for the two speakers are identical. I don't believe there is any big difference in the electronics and have never heard of the Ascent i having the NAC tweeter. That is generally found on the lower end speakers like the Clarity.
You would probably get a lot more bang for your buck by investing in good amps than by doing the upgrade to the i version. I have found with my Ascents that the performance upgrade from biamping them with good amps is phenomenal. A quality solid state amp for the woofers and a powerful tube amp for the panels will make these speakers sing like you wouldn't believe.
With that said, these speakers are starting to get a little long in the tooth, so a panel upgrade may be beneficial depending on the condition of your particular panels.