I've used the Motion 4i for awhile now with my ML setup (similar to yours, but I have the 60XTs up front, and Motion FX2s as surrounds. If you can mount another set of LX16s, that would be ideal. But if they're too heavy or too deep, the Motion 4is should fill the bill, especially if your room isn't massive. Cross them high (like 150-200hz) on your AVR. They're light and shallow, so bass is anemic.
I also considered replacing them with something more robust like the Ascend Sierra Lunas since they use a similar ribbon tweeter -
http://www.ascendacoustics.com/pages/products/speakers/luna/luna.html
I've also used the Polk OMW3s and SVS elevations in smaller secondary room dedicated to gaming, and the Polks surprisingly aren't bad. SVS were only slightly better, imo.
If I were in your shoes, I'd go cheap on the atmos, and allocate any funds towards another subwoofer, or perhaps a bigger sub. If you're perfect happy with your bass, then I think the Motion 4is are a safe bet.
Good luck!