I've now also completed my little modification
and I'm quite happy with the result. All devices have gone to 5* signal on the heal report, where previously I had a number that were only 1 star.
Personally, I'm not too bothered about the warranty issue (but that's just me). I've had my Vera over 12 months, so I'm not actually sure if the warranty still applies!
The modification is pretty simple. I've attached a picture of my finished product (with the top removed). You can clearly see the z-wave board and the new antenna. The internal antenna is glued to the top of the Vera case - I taped over the connector to electrically insulate it, and then taped it to the top of the case (being careful to avoid covering the ventilation holes!).