Okay, i'm gonna post the way I did cp_banana and did massive changes quite quickly. I believe that everyone does rotational/mirror symmetry wrong.
Make half of the map as you would normally, place all the props and lights and entities and all that jazz too. When you are ready to mirror the map, make sure the mid is pretty symmetrical and slice it down the middle, deleting 50% of it.
Then you need to select all the stuff but gamemode logic and such, and copy it over to the other side.
Once this is done, change all the triggers and stuff to be compatible with the other team
(Use UEAKCrash's ancient mapping technique for this).
Once this is done select everything on one side of your map, and deselect entities/team based stuff, make a visgroup called "Red geometry" and then do it for the opposite side. Do the same for the entities, giving them each their own visgroup per team.
There you go, now if you want to drastically change geometry for a certain area all you have to do is do it for one side, turn off visgroups for the entities, delete the opposite sides geometry, copy/paste/mirror/rotate your selection and turn your visgroups back on. If you need to change textures, keep a notepad file with all the equivalent textures for each side and then use the replace textures button.
If you're changing entities do the same thing just with the geometry visgroup turned off. Enjoy!