Panasonic 14mm w/ & w/o GWC1 wide converter

The Panasonic DMW-GWC1 0.79x came into stock at Adorama and I picked up a copy to have an inexpensive 11mm in my kit. I haven't had a chance to do any proper shooting with it, but I did take some comparison shots of my handy test target using the Panasonic 14mm f/2.5 with and without the GWC1.

First, some quicky iPhone shots of the converter and the E-M5.
Adapter with its cap on the camera, converter with its push-on caps alongside:

A shot from the side so you can see the additional thickness added by the adapter and its cap:

The adapter itself functions well as a lens hood with the converter removed:

Converter attached:

The converter attaches securely to the adapter. The adapter screws into the filter threads on the lens so it has a tendency to come loose a little when removing the converter. Panasonic neglected to include filter threads on the converter.

Now the comparison shots. All taken on a tripod, IS off, with 2 second delay. I tried to match the framing between the two sets of shots, although I obviously didn't quite succeed. I made no attempt to correct for possible field curvature. Processed in LR4.1 at default settings, exposures equalized (they were very close to begin with), and chromatic aberration removal ON since that's how I normally process. Crops are 100%.

The "test target":

Center crops first:

Corner crops — first set:

Corner cops — second set:

I'll try to follow up with evaluation once I've had a chance to put the lens to real use. However, this comparison suggests to me that contrast loss with the converter is visible but fairly minor at center. The corners on the 14mm aren't great to begin with, and with the converter they look worse stopped down than on the 14mm alone wide open. At f/2.5 the corners don't have much resolution at all, and f/8 already exhibits diffraction softening with little gain in corner performance. I'll probably just "set it and forget it" at f/4 or f/5.6.