The braces are designed to ensure that the studs into which the rear subframe mounts are held in double-shear rather than single shear.
"IF" you are going to remove them, then the race engineer in me would want to periodically inspect the studs for cracks.
Your cage supports the bodyshell but does not directly reinforce the stud into which the subframe mounts.
If you are replacing, or have replaced, all your suspension bushes with solid, then all lateral/longitudinal loads (including shock loads from hitting things - like kerbs) are fed undamped immediately and directly into the subframe and in the absence of the braces, the studs which secure the subframe will see a bending moment which will eventually lead to fatigue in the stud.
Whether this is a fatigue load that is sufficient to warrant any concern I cannot say, but simply observing that the Nissan designers thought that double-shear mounting is required for road-car durability tells me that it is probably worth retaining.
OEM's don't add unnecessary weight to their cars without reason.
I'll simply observe that I have had some suspension breakages in race cars "at speed" over the past 40 years and not one of them was a pleasant experience, personally or financially. Personally, I'd take a guide from the NISMO RC - if they have them, then they are needed.
Hope this helps make a decision - it is up to you at the end of the day.
RB
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