Both sides [;)]?
There's only one (1) side to this.
Why would one need an "ATF Letter" for something that's specific in the law. Also in the ATF FAQ with references to Federal Code. There are no "grey areas" here.
You can't create a "semiautomatic assault weapon" after 13 Sep 94. Pure and simple.
A short barrelled AR15 manufactured after 13 Sep 94 is a semiautomatic rifle with a removeable magazine and a pistol grip. That's all the evil features you can legally have.
Barrel length has nothing to do with it, you can't create a short-barrelled semiautomatic assault weapon either.
If it was as simple as paying the $200 for a tax stamp "pre-ban" receivers wouldn't cost $600 and up, we'd all just register them as SBRs and load up on the evil features.
A SBR AR15 rifle with a very short barrel has very limited utility in 5.56mm, although one in a pistol caliber would be handy at short ranges.