Some good news. Just checked 16mm with the UWA converter on Nex7 - no purple shift to talk about, merely a few percent lower green wrt red/blue in the corners, essentially unnoticeable.
Corner smear is there as expected, need to crop by 1.25 to cut the smear off. So altogether it makes a good enough 15mm f/3.5 APSC at 15Mp res, but noise is also 0.7EV stronger due to crop.
Good work! I haven't had a chance to get back out with mine to see the effects, but that's good to hear.