The open-source Nouveau driver does indeed work with the GeForce GT 710... On an Ubuntu 15.10 box, the GT 710 lit up fine, albeit I ended up moving to Linux 4.5 and Mesa 11.2-devel for the best experience. The GT 710 hadn't regressed and was still working out on this newest open-source code.
For today's tests, I compared the performance of the GeForce GT 710 on the NVIDIA 361.18 proprietary driver against Nouveau with Linux 4.5 and Mesa 11.2-devel Git. On the open-source side I did two runs: one out-of-the-box and another when manually re-clocking the card to the highest performance state.
In some tests, when manually re-clocking the GT 710 the open-source Nouveau driver performance was very close to NVIDIA's binary blob.
In other tests, the performance was still well behind NVIDIA's proprietary driver.
To see more of these open vs. closed-source driver benchmarks on the sub-$50 NVIDIA GeForce GT 710, see all of the data via this OpenBenchmarking.org result file.