The down fall to using the jumper board, is that you won't get all the options that the XFI has, unless you repin the connector and run those new wires to the new options (without going thru the jumper board). I think that would look pretty messy. The jumper board is a great idea for dyno testing, but I am not sure I would put it in a car and run it. You would basically have the same options as the older system with the jumper board, if I am not mistaken. Besides, I have seen those jumper boards, and they don't look water proof enough to be in a car, but nothing is totally water proof, lol.
If I were going to go get a new XFI system, and I had an older FAST box, I would just rewire the car, versus the jumper board option. Just my 2 cents.