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Hi Ben Seba,
We strongly suggest you upgrade. 10.1 is out of full support.
I've never seen an impact. Admittedly I haven't worked with Nessus 7.x yet so that's based on <=6.x. I can't see it would be much different though.
If auditing is enabled (STIG scripts) you will potentially see a lot of additional activity that may look like intrusion but isn't. Of course, if the security team are using brute force options to get in accounts may be locked out
If there is a concern about a performance hit, then run Nessus against the appliance in periods out of scheduled scans.