One thing I could see being a problem with a tub is providing the basking area. It may get too hot near the lamp and maybe melt the plastic? See if you can rig it up and if it works then I see no problem in keep him in a tub.

My beardie male was 18 inches at 8 months. He was kept at precision husbandry and care though.
12 inches sounds about right for the normal care a person would give. I've seen worse.

Just make sure your ready to provide the right care for the beardie and you'll enjoy having one.