There are lots of color option for the hair and they are all 2k and 4k size maps which is quite normal. Im referring to the tex folder size btw.
As for torment horn they are indeed the best looking horn out there. Its just the size is ridiculous. I also counted 10 4k texture maps inside torment texture folder just for the horn alone.
Having said that, as you already said, there could be other factor which im afraid i have no clue at all regarding memory leak etc. My system couldnt even get past dforceing the hair down :p