vray: Can we use 4 times small resolution irradiance map to generate final render image?

We need to render hundress of frames of a very complex scene render using gi and irradiance map in 1280x720 resolution. Most of the frames are taking around 4-6 hours per frame in render farm (one of the top/reputed online render service provider).

To save the time, I remember some one suggest to do first animation prepass to generate vrmap (irradiance map) in a smaller resolution and use it in animation render using the previously generate vrmap and also he said it is acceptable and recommended to generate the vrmap in 1/4th of the final bitmap resolution we want. So in my case it will be 320x180. Is it ture? what is the least acceptable resolution we can take for generating a 1280x720 render/bitmap image?

we are using 3Ds max 2014 and vray