996/7 GT2/3 use a similar bar setup for the subs
Here is how the factory did seat installs on 964/993 RS/Cup
#22 the tunnel side harness mount and #18 spacing washers are placed between the slider rail and the sidemount. the sub bar would be similarly sandwiched.
But of course do check w/ the sponsoring organization.
The feedback that I have gotten varies from "crap" to "not ideal but it does satisfactorily solve a problem" This from scruts and pro race team engineers
some cars, like Corvette must do it this way.
I'm still considering it for mine.