I've found this comment by surfing on skyrim nexus:

"With time and a bit of patient. You can use BSAopt (here in nexus) to open vanilla (and official DLC) .bsa. Then you have to copy/past into "textures" folder the textures of your mods, overwriting when asked. Finished this step, you have to create one or more .bsa file using the "archive" program of Skyrim Creation Kit. Each .bsa shoulnd't be over 2GB (I use optimizer textures v.0.83 -here on nexus- to have good texture performances. Strongly suggested).
At the end, you have to change the Skyrim.ini files, addind under [Archive] session new .bsa. Be careful=the exact name, or the textures won't be loaded.
For last step you can find a lot of licterature on forums (nexus, tomshardware, bethesda, and other).
Have fun with your 10-12 fps gain "

Now, I have 2 questions. First of all, has anybody tried the above quoted procedure? And in case, can anybody explain it more in detail? I know how to open .bsa files and how to create new archives with the creation kit, but some of the passages are unclear to me. Why should I open the official bsa and add the mods textures above the vanilla textures? Does it mean I can delete the official bsa after creating the new ones??
EDIT I know it sounds strange to report a comment in another forum, but it's quite old, so I can't ask directly to the author. Also, I couldn't find anything similar by searching on internet... Since my texture folder is around 5 gb, I'd really like to compress it into a bsa if that meant gaining fps...