Can I render the irradiance map at 400x400 and then use it to render a 800x800 image? What will be the effect?

you may get artifact with the render, check chaosgroup videos on youtube. I don't remember the exact one but in one video he showed how can have like3 to4 maps around your structure and then vray will automatically combine them and you will be able to produce faster renders with less or no artifact. 

Your question is not clear. "400x400" mean what??

Is this Output Size WidthxHight?

Or its Render Region Division XxY?

Yes you can to cut time short, but sometimes it will give you some noise in the final render specially in large scenes. I prefer to use irradiance map of600×600 with (dont render final image) checked in your case.