Hi Jimbo....now that has nothing to do with low production numbers then ...does it. If you produce say 5000 of the Olds bumpers and 750 of the as you say identical Caddy bumpers, the production cost would be based on a total production run of 5750 vs cost of tooling ;-) The price difference is marketing.
I remember the same thing from the Audi/VW dealer where my buddy worked back in good old Germany. The Audi customers got to pay 25% more for the same part out of the same parts bin :-)
Not to mention that my buddy got really good at "costume changes" when going from "Specialist Technician VW" to "Specialist Technician Audi". He still is the best I know and the only one besides me allowed to touch my Alfa GTV 625 3.5

Even if Borla were to manufacture the exhaust with tips more like the stock ones the difference in tooling would be small.
But it was always a little more expensive to have something special.

The $900+ would be acceptable to me if:

1. Looks exactly like stock XF
2. Looks exactly like stock XF <-- really important :-)
3. Improves performance even if only slightly but definitely no loss allowed.
4. Gives a slightly more aggressive tone without being unnerving on a longer drive.
5. Is made out of stainless material so the investment lasts a while.

Based on my list of 5 you can easily see that this would be a purchase I would only make after being able to lay hands and ears on it. :-)