There's a trick to reducing refraction noise, but it's super annoying to implement. If you disable refract on all your lights, then the refractive object will no longer directly refract them, and will not split rays sampling lights. However refractions of your environment will no longer appear. To make up for it, you can make emissive objects that are visible to refract only. This is incredibly annoying with cube map textures, but not so bad with a typical lat-long image. Most other renderers I have experimented with won't even allow direct refractions, and require setting up emissive surrogates in this way, notably arnold and redshift are both this way.
Another setting that contributes to the noise greatly is dispersion. I know it's pretty, but it very expensive in terms of sampling requirements for low noise. In your test scene, the values you are using were also extremely high, physically plausible values are between 0.001 and 0.05 depending on the IOR. If you simply like the look, then by all means, go nuts.
Wow! that does sound complex I will give it a try. I am really focusing a lot on refractions because I is the only area in Mantra I have problems, and I am a bit of a hardheaded type that will not give up .
And thanks for the tips on dispersion aswell….