I remember editing the water/splashes for my Hell map for h2h years ago. Been a while.
I don't think HiTech would approve of them simply because 256x256 is much much larger than they presently are. They may not hurt your FR for that size terrain in H2H. But in the MA you have skins (with resolutions of 512x512 or 1024x1024) and many more players to contend with. If you reduced the size to 128x128 it would reduce the download size by 1/4 and there would not be a major loss in quality.