I think MeshCAM Standard will do everything you want. You can load an image and automatically convert it to a heightmap similar to what Rhino does. You can optionally smooth it if it has noise or characteristics that would make it difficult to machine. The toolpath part is pretty simple- you can generate a parallel finishing toolpath (and a roughing if necessary) and it should calculate very quickly.
I can't speak to what the other programs you mention do but it's worth noting that MeshCAM treats the heightmap as a real surface to be machined. It will calculate the toolpath by offsetting the tool from the surface like it would for a normal CAD file. I've seen other programs that just drive the tool across the image surface with no offset or interference/gouge checking so a few bad pixels can ruin the output.
I believe that Photo-VCarve would be similar to the simple image machining workflow in MeshCAM Standard. MeshCAM Art would be more similar to Aspire where you begin with the art/photo and then build your relief. (but MC Art is much more limited than Aspire)
You'll also get all of the other 3D machining capabilities that MeshCAM has. Maybe that would be useful for you at some point.
-Robert