Actually, it will help tremendously even with purely diffuse surface. Let’s say you have a strong sun light where small streak of light reaches deep part of the environment. If the sun is really strong, then variance of samples returned from that are will be significant.
Another very common case which can cause noise without ray clamping even in diffuse scenes are small and strong light sources very close to the surfaces.
On top of that, while I don’t know how your GPU path tracing works, generally, GPU path tracers do not do ray branching, and they also generally don’t support some irradiance/light caching for secondary bounces. In that case, having ray intensity clamping will actually help you that much more! 
I have created two examples of the cases I’ve described above using offline renderer (Corona). All the offline renderers these days have ray clamping by default, set to some reasonable value (usually around 20, but for purpose of game engine, I think even 2 will be fine).
Here’s example of the sunlight case with ray clamping values:
Ray clamping OFF:
Ray clamping 20:
Ray clamping 5:
Ray clamping 2:
And here is example of the other case, small strong light source near the surface:
Ray clamping off:
Ray clamping 20:
Ray clamping 5:
Ray clamping 2:
So I definitely think it would help you a lot
EDIT: You may want to right click and open the images in a new tab to see it better without excessive compression