In past tests, I've found that increasing the gain in HD seems to be OK up until +12 dB --> +18 dB of gain. (Depends on a person's willingness to put up with the noise pattern.)
However, when I convert the 1920x1080/30p HD video to 640x480 SD video, 9 pixels combine to make 1 pixel. In theory, this should help to reduce the impact of noise in the video.
No question that it helps, but it's subjective as to how much more you can push things. Some may feel that +18 dB should be the upper limit, others may find that +24 dB still holds together very well, and a few may be willing to push to +30 dB.
In order to avoid any extra compression noise getting in the way, I compressed this video at a very high bit rate, thus the large size. You can download the fie and judge for yourself what the upper limit should be when converting to SD resolution.