Well the statement that A&G elongate the chin with a maximum of 4mm with sg surgery appears not to be true. I like this result a lot. (Arnett & Gunson, found this picture in the following thread, didn't find the exact source though: http://www.jawsurgeryblog.com/forums/viewtopic.php?f=8&t=3628&start=30).
This is an amazing change. I'd love to see more sg results of A&G. The guy underwent orthognathic surgery too though of course.
Actually it is true even though I disagree with their philosophy on it. The chin point itself which was seems to be what you're referring can end up 40mm if needed (I don't think I've seen much more than that but it would be fore people with basically no chin) from where it started mostly through a large mandibular advancement which is what A/G can do. The cutting of the chin bone itself to be advanced aka sliding genioplasty is where the 4mm max figure comes from.
For what's worth, I had a 8mm genio before having double surgery with Gunson where they advanced my mandible around 13mm and it wasn't an issue. I don't know if it would have made much difference if I never had the genio and they lengthened my mandible a bit more then done a smaller genio.
I think the main reason they sway from larger genios is that in the past doctors would compensate for the inadequate mandible advancement with large genioplasty. Also that step-off deformity may result from genio's although I believe that can be dealt with during surgery with some sort of augmentation like HA or implants. But anyway once counter-clock rotation was utilized, the occlusal plane abnormalities could be corrected and this allowed huge jaw advancements with stable results. In any case, I don't think it would be necessarily "poor planning" as they put it if someone had slightly smaller BSSO and larger genioplasty as long it didn't compromise the final result.
And OP not sure if you meant doc's who do just genioplasties or in conjunction with jaw surgery but A/G don't do isolated genioplasties.