In light of the recent tour de force of isotropix clarisse performance demos as well as my own demo trial of clarisse I tried to replicate the same situations in Houdini.

I tried to fill the scene with hires models and various instancing and duplicating options. The bottleneck wasn't rendering though (except with updates when objects had displacement), but 10X more the viewport performance.

In my opinion sesi should add the rendered view mode for the main viewports and allow to turn off effects that dont update quickly and/or need long precomputes. Then overlay opengl manipulators on top.

Also the refinement could be more sophisticated. I remember reading a graphic paper few years back that interpolated the samples with progressively subdivided triangles that had gradients as sample interpolation and the contours were weighted more than interiors giving good silhouette on low samples. Anyways the point was getting the feedback fast.

For high res scenes this would work better than opengl at least thats what I saw in my tests yesterday.

you are talking about very limited number of cases, where your geometry doesn't change, so pretty much just camera, lights object transforms
as soon as geometry changes, it is certainly much slower to update

so instead of adding rendered mode to viewports, which you would keep almost always off, it would be nicer to have selecting ability, camera navigation and handle overlay in render view, for some minor tweaks
even though having another viewer with display proxies opened next to render view for such tweaking isn't limiting for me, maybe just in single monitor situations

but if houdini and mantra had shared memory and conversion step wasn't necesarry, it might nave been very nice speedup for realtime tweaking even for changing geometry, I'm not sure if that's easy to implement though