In general, you might also want to make it work more like the lightpath trick seen here.
http://cgcookie.com/blender/2013/01/14/reducing-caustic-noise-blender-cycles/

This would mean that disabling caustics in effect will not remove near as much realism as it does now, which would then serve as a good compromise for a lot of people doing things like animation.

Regarding TODO 2 :

• backward compatibility can be solved with version patching in blenloader/intern and a version bump (no_caustics maps to caustics_reflective = caustics_refractive = false)
• if you also want to preserve upward compatibility, you could keep the old no_caustics variable in DNA and add a get/setter, which updates it accordingly, when one of the other variables gets changed (and leaves you with the interesting question, what to do with the mixed cases :) )

the first you should definitely do, the later is somewhat hacky...

It's not a DNA value, it's only available in py. But I just found out that we have a version patching system for that too (just added by Sergey a few days ago). See intern\cycles\blender\addon\version_update.py That should do the trick. :)

it is also available in DNA using properties API (everything in blender goes into DNA, there is no other way that data can end up in a blend file :),
but Sergey's new python version patching for cycles looks like it is a lot easier to read and maintain, so go ahead :)

Did a few test renders of my own. Glad to see this functionality added so quickly after I mentioned it! These were done on 6 of the 8 cores on my 4790k. You can see the big time savings in scenes where you need reflective caustics, but where you can stand to not have/fake refractive ones.

Hey @Brecht Van Lommel (brecht) and @Sergey Sharybin (sergey) :)

I am not 100% sure how to make this work for OSL. I checked and it seems OSL distinguishes between scattering and event types. See https://github.com/imageworks/OpenShadingLanguage/blob/8ca9327413f3179f4be9c2714c533c705ad3e07c/src/include/OSL/oslclosure.h for example (Labels class).

Do we need to add a m_event_label, next to m_scattering_label in CBSDFClosure()? Some hint would be appreciated. :)

We don't use those labels from OSL, only our own LABEL_* flags, which are bitflags that you can combine. In osl_closures.cpp, bsdf_phong_ramp.cpp and osl_shader.cpp you can replace LABEL_GLOSSY by LABEL_GLOSSY|LABEL_REFLECT and LABEL_GLOSSY|LABEL_TRANSMIT depending if the BSDF reflects or transmits.

By the way, the light path trick mentioned above is like a more aggressive version of filter glossy. Filter glossy will increase BSDF roughness up to 0.5, this trick replaces it with a diffuse BSDF. Filter glossy could be changed or made configured so it can go all the way up to 1.0, especially with the new glossy BSDF sampling that is less noisy for high roughness. Roughness 1 glossy is not quite the same as diffuse, but it's a very soft reflection too.