Unfortunately, the 6450 or GT630 are not very high performance cards, but they do seem to be the fastest things you can get with a low profile passive cooler. I think the GT630 is the faster of the two, and personally I prefer Nvidia. I have a GT610 I picked up as a backup card (cheapest I could find with an HDMI output in a pinch) and it will run madVR on the lower quality settings without too much trouble.
The fastest "standard" low profile card I've seen is an AMD 7750, though it is not passively cooled. It should be quite a bit faster than the GT630 though.
There's actually a very strange looking low profile 7850 available, but I'm not sure how quiet that will be.
I just added the Asus passive-cooled Radeon 6450 in my low profile case, and I can now run ROHQ with DXVA2 image upscaling and downscaling, and SoftCubic chroma scaling, and nothing checked in the "trade quality for performance" list (except for subtitles, since I don't care much about subtitle quality). (The color using SoftCubic looks very good, and since that is what is being scaled, it seems fine to my eye.)
I would suggest using Mitchell-Netravali rather than SoftCubic for Chroma. SoftCubic has a tendency to desaturate colors. (actually, MN does as well, but to a much lesser degree)
I don't know how DXVA scaling is with AMD, but on Nvidia it's actually
more demanding than the lower-end scaling options in madVR (ROHQ) and looks worse.