Record lossless, transcode the files you'll be using to an intermediate codec, in this case DNxHD90 if there's a ton of movement in the clips. Delete the files you won't be using.
It'll look slightly worse, because it's still restricted to record a certain amount of pixels, and 1920x1080 doesn't cover the screen. By changing the resolution in-game to 1920x1080 then recording, is giving YOU a worse experience but not your VIEWERS, as it's just capturing everything 1:1 pixel mapped. If you have any questions or need help to make this happen, let me know.
All the best!