Quoted:
My question is: is there there any reason I should not use the conversion kit and instead pay extra for a dedicated SBR lower?
Do you mind paying the extra tax? I've considered registering another SBR lower just so I don't have to switch uppers / mag blocks around. With a dedicated 9mm SBR lower, you can either get one of the dedicated lowers (like those that use Glock mags, for example) or you can use one of the types of mag blocks that you need tools to remove (Hahn top loader, I think?)
For myself, I use a single SBR lower, and a quick-to-change Hy-Tech mag block. I have a pistol lower that I keep my "spare" upper on, but almost never shoot it. More than anything, it just gives me a legitimate "home" for my extra upper, avoiding the whole "constructive possession" thing.