I was a little surprised too about the conversion to sRGB when "Printer Manages Colors". Why? I think it's to protect the color management challenged out there, who don't know what they're doing. But what about those that do? What about ABW, as someone said in that thread? It means that everything you send for ABW printing needs to be in sRGB if you want to avoid Photoshop converting it. You can do this, even if your image is in Grey Gamma 2.2, by converting to AdobeRGB (essentially the same thing, but in RGB), then assigning to sRGB. But it's a more complex workfow. Same applies if you want to tell the driver that the image is in something other than sRGB, like AdobeRGB, and get it to handle the conversion (not something that I'd recommend, but not entirely silly).
In that Online Photographer Blog thread, Dave Polaschek is from Adobe so I take his word as authoritative and Andrew Rodney is well-known here, and the same comment applies. They were quite definite about what Photoshop does in Windows. What LR does was less clear, IIRC.
I've done a little more research and Printfab recommends that you operate as I suggested - i.e. set the printer profile to be the document profile - and that you ignore the warning from Photoshop. It seems to work, from my testing so far. Perhaps a safer option is to print from Qimage, which allows a color-management-off workflow.
