The current version is actually DxO Nik Silver Efex 3, not 2. AFAIK, this is the progression:
* last independent-Nik version to free Google version of Nik tools: no changes;
* free Google version of Nik tools to DxO Nik version 1: bug fixes and updates to work with latest operating systems and editing software;
* DxO Nik version 1 to DxO Nik version 2: additional presets and maybe expanded controls; and
* DxO Nik version 2 to DxO Nik version 3: tighter integration with certain other software, including some ability to work semi-non-destructively with Lightroom and Photoshop.
I had the free Google version and I've bought DxO Nik versions 1 and 3; I did not buy DxO Nik version 2.
Whether there are improvements between the last independent-Nik version and DxO Nik version 3 that are sufficiently worthy to you to warrant re-purchase is something only you can decide. Obviously my judgment has been that there are, but that judgment has been affected by both my desire to see DxO do reasonably well financially and my desire to see at least some continued development mainly (like you, I guess) to Silver Efex but also to some of the other tools.