thanks for the concern. you got me checking and according to the site, the safest placement is the rear center as i learned before. consistent with other recommendations too.
The reason that MOST child carrier seat manuals specifically forbid that kind of installation is not because it's in the middle, it's due to the LATCH system and the fact that it is designed with an 11" spacing between anchor points. The FX doesn't have a center LATCH position, so by installing the seat in the center position you are using the inner anchor points from each of the two side positions. Using the inner attachment points has two flaws:
First, in the event of an accident force would be applied to the anchor points in different directions than they were designed for, significantly reducing their load bearing capacity. For an example, try pulling a pencil in half. Now try bending it in half.
Second, the seat itself is designed the same way; To resist force in a very specific direction.
Almost every modern child safety seat utilizes the LATCH points, and for good reason. Because it is far superior to the system it replaced (seat belts alone). For the same reason, every modern car now comes equipped with LATCH points. If your child seat doesn't utilize them, I'd get a new seat. If you are using the LATCH system in the center position, stop. It's very dangerous.
EDIT: Your seat does utilize the LATCH system....
DOUBLE EDIT: I couldn't find the actual installation instructions. It has the capability of using the LATCH points for sure, but I don't know if it's required by Recaro. I do know that the FX does not suggest mounting a car seat in the middle if you do use the LATCH points.