The only way to find out for sure is to try one. As to the weight, it would be negligible. This device is made of quite thin plastic in a circular open form which acts to shape and direct the sound waves inside the guitar body. It would probably weigh about the same (or less) as one of those rubber thingies that some people put in the sound hole to prevent feedback from occurring when playing plugged in. It would also probably weigh less than some of the "in the sound hole" pickups that one can mount on a guitar.

According to magazine reviews of the device that I've read, it improves the acoustic sound noticeably, but it has an even greater beneficial effect when you're plugged in with a pickup.

Seamus - I emailed them and it appears to have gone through. I'll see if an answer comes tomorrow.