Hi Calum, I'm not the developer of this airport, but since Orbx first started creating airports for Fsx and P3d, 5m mesh setting has always been the one scenery 'rule' we have for correct display of the scenery, all developers worked towards this one standard so that all scenery displayed as intended.
I never personally noticed any performance issues with using 5m, but of course each users PC is different, and each user has different priorities visually.
Looking through your settings one thing stands out to me and it's the 'dynamic 3d autogen vegetation' option, this has always been a heavy hitting scenery element that (in my opinion) doesn't look very good as it produces very bright looking trees that visually clash with most orbx scenery, I highly recommend unselecting this (you will need to restart the sim for it to take effect), and setting 5m terrain mesh, and see how you go, hope it helps
Cheers, Tim