If you already have the SBR, why not run a .22lr conversion?
$200 for the conversion bolt and mags and you're done. Bravo, Black Dog, CMMG, there are a lot of options out there these days. There are even complete, dedicated .22lr uppers out there that can be had for what it would cost you to rig up a 715P with the SIG brace.
If you haven't been through the 715T sub-forum already, take some time to read through the troubleshooting threads before you make a decision to buy...