Just in case you hadn't seen/heard about it yet... there is the FLOSS program xrdp. It is basically a service that uses rdp for two end points... but on the remote system there is a VNC shim between rdp and the host.
I believe you can do a single application with it or a complete desktop.... and even if not... who cares. You can have the whole desktop with your single app running. If it is more efficient for the client side that's all they care about. The remote side has the whole desktop but you can make that be a very light-weight window manager if desired.
Now, with a full desktop in the window you can run lots of applications and the speed on the client side doesn't really change. In fact there might be times when you want to do just that. Open up 3-10 different X applications and see how well that functions. It'll be way slower. I'm sure you're counter would be... "who wants to do that?". Ok, you are probably right... but I'm just say'n. :)