Mike - not sure if you got the info your looking for on the 300 Whisper? I don't have any experience with them, but I do have a book with several whisper cartridges written up - reloading info type of thing. The 338 and 375 whispers sound kind of interesting as well. If you want to see what they have written let me know.

Oh, and as far as the 243 goes - seems like243's are always anticipated to be no-recoil guns, so recoil pads are often hard plastic. If your going the 243 route with a young shooter and want to assure minimal recoil - make sure there is a recoil pad on it. My 6 mm makes my shoulder hurt more than my 7 Rem Mag. The 6 has a hard plastic butt pad, my 7 Mag has a custom stock and a recoil pad...

It would be interesting to see what shooting a subsonic caliber would be like. The book talked about these calibers being incredibly accurate out to 1 mile, and how little wind effected the path with the heavy calibers and bullets. More so with the 338 Whisper and larger than the 300 Whisper.