Have you tried bagginbills lightmeter? That will solve your light (intensity) problem. You do know that you can increase intensity to over 100% ?

I am not sure which seams you mean: If you mean the shadow artefacts of the room - I think you can solve them by increasing the IDL bounces, but others might know more about it.

If you have seams on the texture: Turn of texture filtering on the texture node or lower your shading rate