IR photography questions

[Harry Lime]

I plan on shooting with an IR flash / film set up for a project, but have a few questions.

Here is my set up:

Metz flash with IR filter.
Leica M (ttl) with Efke IR820 or similar.


If I am using something like Efke IR 820 Infrared Film or the discontinued Kodak HIE is it imperative that I use an IR filter over the lens of the camera?

My thinking is that if I use an IR filter over the lens, then the film will only pick up illumination from the flash gun.

If I leave the IR filter off the lens, then the film will pick up mixed light.

Does anyone have any experience with this?

thanks