Odd. As shown above I had zero signal impact. Field test showed no change with the case on vs off.
Have you tested with the iPhone field test to see real impact?
I drove with GPS today as well and no issues.
I haven't used field test, but can easily reproduce the issue with this case. It is manifest mostly where the signal strength is already weak (remember the iP4 issues...).
Speedtest confirms the problem:
same room as router -- 20mbs with or without case
2 rooms away - 5mbps without, 2 with case
4 rooms away - 1mbps without, doesn't connect with case
Is yours the same case?