I would be very surprised if the hole wasn't exactly the same as the iMix Onboard. This would almost certainly be a simple mod, since the iMix uses the same Element UST. Of course, you'd have to add the iBeam, but that shouldn't be that big of a deal. I have no idea about the Aura, though I doubt it would be much of a deal.

You should be able to go to the Fishman and Baggs websites and get installation instructions for both. I would assume that they would give you all the dimensions required.

If you're not in a hurry, you might want to wait a little while. Baggs is introducing a new mic (the Anthem) that works in conjunction with the Element UST. I don't believe they have immediate plans to offer an upgrade kit, but it ought to be an easy thing for them to pull off. I'm keeping a close eye on the Anthem; it seems like a very promising development.

Good point about the Anthem.    They're about to release it,  but I'm not sure how quickly will they release the Onboard system (if they will at all). 

My other alternative is basically don't bother with the mod, just get an Aura Spectrum DI.  But it'd be nice to have an Aura  /  mic  / SBT inside the guitar itself.

I hope Bagg's doesn't forget to bring out a upgrade with the Anthem for those of us with an onboard system.  Their current anthem is quite limiting really, you can't have a guitar with braces around the soundhole like CA has.  If so you are out of luck, very stupid to me unless this is just the first confiquirtion they are offering.  Hopefully more to follow.

Steve

Ah...  So an Anthem won't even fit in a CA then?  Because I was sort of wondering if I could swap my Stage Pro in my GX for an Anthem.  Or just have an Anthem in there as well.

I'm hoping they do a soundhole version of the Anthem, but that seems like it might be impossible.

it depends on the bracing of the top of the GX, but i think there should be no problem to install it in it. the problem is what you would do with the preamp, that would became useless. maybe you could remove it, so you have a soundhole for you to listen to yourself more clearly

Not really that simple, if you want to use the anthem preamp you have to fit it under the soundhole.  Most CA's with soundholes have bracing that would prevent this.  There is a simplier version with no blend just a volume wheel but I would rather have the flexibility of the IMIX over that.   I just have to assume that if the Anthem takes off Baggs will come out with other configuirations.

Steve