FinalGlideAus suggests running a GoPro Hero 1/2 lens on your FPV camera--and i agree. The GoPro lens gives a field of view in between that of a 2.8mm and a 2.1mm lens, and has great overall image quality. But why not just use a 2.5mm lens that's designed for the image sensor in the camera? The answer, as I think this video shows, is that the 2.5mm lens has very unpleasant distortion around the edge of the frame, whereas the GoPro lens has almost no distortion. I think one possible reason for this is that the GoPro is designed for a much larger image sensor, so the FPV camera is using only the center portion of the lens, where image quality is higher and distortion is lower.
This is the exact GoPro lens that I buy for my 1177 cameras.
Please be aware that FGA says that GoPro Hero 3/4 lenses don't work, although I haven't personally verified this.