It makes sense for vehicles to have an interior, Vlad.

For it allows the TC to do more than just stare out the optics, he can look at things like the IVIS which gives him a better situational awareness.
Also looking through an CITV/Peri is still like looking through a straw compared to using the vision blocks in the commander's cupola.
As for the gunner, well not as important but it could provide immersion, by having a non-hud way of showing weapon states.

As for the Performance, well the bradley in Arma 2 had one and the performance wasn't bad.

Aside from that, look at the new BW Mod, they've made some good gunner, driver and commander's position.
For the driver they made a quick and dirty solution by just adding additional viewports in that mask.

Also look at the old tank simulators of the nineties, their interior was 2D often clickable, but it worked.
It didn't require a lot of performance but gave you a better situational awareness and immersion.

How about Gute Jagd? Though that is more in an airforce sense.

Or "Hals- und Beinbruch, kommt mir in einem Stück zurück!" ;).