Well it looks to be strong but as mentioned it does pick up the Isofix points which are only really designed to hold a child....but as you mention JME the load would be spread across the 3 Isofix points and the 2 seat belt mounts. I wonder if the Isofix can still be used with the bar fitted?

I have read very good reviews of the CG lock if you dont want to go to the effort of a harness or the compromises it can bring:

http://www.cg-lock.com/

I think something else to keep in mind is that you are hoping to never need to make use of your restraint system so if your just in a road car you might want to wind it back a few tenths and rely on the harness just as a means to keep you held firmly in the seat