9.5mm is plenty. I think 12 would be too much. Mine fits perfect in a new hose, but if the hose has been stretched (like via a mouthpiece), you have to shove it in a bit further in.
IMO, if your going to order a downstem from Mike, you might as well save on shipping and get his adapter. I'm sure he would change the angle of the bend to whatever you would prefer.
I've been using the SSV adapter more lately, but that is because of how my SSV is set-up.
The shortest distance is a straight line, so to use the least amount of hose you have to get the bubbler's top even with the SSV's Heater cover. The hose in the image is only 7.5" long. This eliminates much of the priming in takes to milk your bong, I highly recommend it.