Yes, this is Airbody's reply.
"Signal loss is really complicated. It depends on your signal strength and signal direction.
Normally when you have good reception, signal effect by an aluminum bumper will be minimal.
In the other hand, if your already have bad reception signal loss will be more noticeable.
Airybody aluminum bumpers does effect the signal a little, but not much.
We tested with iPhone 6, the result was averaged about 3-5 percent signal drop on both wifi and LTE.
What's interesting was when we flip the phone around or turn the phone in different direction, the signal strength recovers.
It seems like the bumper altered the direction of signal, not blocking it."