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I've revisited the interior scene, same as before but with a 5000 sample limit.

TYPICAL INTERIOR SCENE 5000 SAMPLES
DIFFUSE KERNEL
5000 Samples, Diffuse Depth 3, Specular Depth 12, Glossy Depth 12 MS/sec = 2.91
Rendertime = 06:47

PATHTRACING KERNEL
5000 Samples, Diffuse Depth 4, Glossy Depth 12 MS/sec = 2.92
Rendertime = 06:46

Miniscule edge on speed again with PT let's call it a tie but my eye says the PT render is cleaner, more accurate, less blowing out of highlights. I'd really need to setup a textured sample scene with some reflections / refractions I think this is where Diffuse might have the edge. But to get to this wireframe stage I don't see the point of using Diffuse it offers no speed gain over pathtracing and PT resolves quicker.

I meant the benchmark scene pt has more noise.
Your interior its not so easy they're both noisy.

STANDARD LW BENCHMARK SCENE
DIFFUSE KERNEL
2500 Samples, Diffuse Depth 2, Specular Depth 4, Glossy Depth 4 MS/sec = 20.16
Rendertime = 01:12

PATHTRACING KERNEL
2500 Samples, Diffuse Depth 3, Glossy Depth 4 MS/sec = 19.92
Rendertime = 01:12

dead heat and they look the same!

They sure do. Have you tried adding displacements?
Now for your interior. Usually pathtracers have problems with enclosed scenes with multiple bounce.

In my interior the rug is using displacements.

Can you upload some screen shots you said your getting a 25% speed increase with diffuse.

Will do. Remember my gfx cards are much slower so the times differences have finer increments so its very noticeable.
Have you tried rendering the grass scene with displacements? Does it make a difference with PT or DL?

Hmm but PT don't have specular depth level (glass stuff?) so in some situations it looks different and even in your test room i liek that Diffuse version a bit better (yellowish sun light inside of room while PT is different tone) so even if is the same render time now it's good to have options. Also i doubt Juan would just remove Diffuse version as long as it's in standalone it's needed in plugin at least for compatibility sake.

I think PT has spec depth but it's tied into glossy depth. With Diffuse you can configure each separately (spec for reflections, glossy for refractions) but no cheating with PT. With my interior the warmer one is actually not what it's suppose to look like according to my lighting setup, it's too warm. This is nothing that can't be done with tweaking the colour temps/intensities or even in post with a few mouse clicks. Also the highlights are out of control check the blowouts around the couch and rug. The point is PT is returning a cleaner image in the same or less time. I'm not suggesting Juan remove the diffuse kernel that's not Juan's call to make in any case. Pre Octane 2.0 diffuse was faster than PT. Maybe with all the new features added it's having an impact on the diffuse kernel more so than the others? Maybe diffuse is still broke? Don't know. What I do know is I used diffuse all the time but now there's no use for it. Since diffuse is a cheat anyway maybe Otoy could maintain this as their 'optimised for speed' GI solution and find a way to speed it up.