Please email us at techsales@opticsplanet.com, so we can further answer your question.
Yes I have same set up
Yes, it will work exactly as expected. Most of my AR15 and AR10 setups have JCR SCS Gen2 and Magpul UBR Gen2 stocks. Although, i use different (JP and Radian) Bolt Carrier Groups but that shouldn't matter in your case. Since Magpul's (included) tube extension is proprietary, Magpul includes a small spacer to make it function as a normal tube.