I don't see the point in that subwoofer, you have the same amp driving two woofers instead of one, it isn't going to get that much louder or dig that much deeper than the single driver version. You would be lucky to get a 3 db increase. Plus, it is awkwardly shaped and not competitively priced relative to other subs you can get for 600.
The E56Ci center speaker, on the other hand, could be a worthwhile upgrade. I always thought that the regular E5Ci was a puny center, especially for the E55Ti towers, and given the importance of the center channel in home theater, I think there was always better speakers sets to go with because of that weak link. It would be worthwhile to compare the E5Ci and E56Ci in your review, to see if the performance increase justifies the price increase. I also wonder how the E56Ci and the Infinity C351 compare, their drivers are the same size and are arranged in the same way, I think that would be an interesting comparison.