I recall having all sorts of problems when I updated the T3 System plugin some time back.
Version 2.6.5 works fine on all your templates with T3.
However, there is a new version 2.7.0. for the system plugin (I may have skipped a few updates for fear of mucking up the site).
Have you heard of any issues from those who may be running the latest T3 System plugin on any of your older templates?
I like to keep all sites up to date, especially as Joomla keeps evolving, but don't need the headache when things go wrong with T3 system plugin.
Would it be safe to update the system plugin?

Hi

I'd leave it for now

We are running an audit of all the templates and organising the new development team ready to start working in about 10 days once holidays are over

I've not heard of any problems in updating but I can't think of an immediate reason why you need to update

Cheers
Paul