Well, I finally tested the piece of software made by Harkaz. For those who are new on the forum, harkaz is a computer expert. There have been some articles about him on the internet and he's also present on the forum Ryanvm.net.
For years I have made a French updatepack for xp over there too, but I left it and after a few years of absence on the internet, I discovered this forum.
When I decompressed the archive of harkaz my antivirus found something. Since I was using the x64 version I didn't care. It's probably a false positive anyway.

To use rebase it it's quite simple, even though harkaz made a very detailed tutorial which is included in the archive.
You just have to launch RunAsTI64.exe, go to the path of the exe and then use the command line "rebase -online", if you want to reduce a running system.

After a few minutes, the huge winsxs will be trimmed down.
On my system this folder lost 1,5 gb and I had already removed a few Gb of stuff.

If you want you can contact harkaz and buy his program. And you can use it as an opportunity to make a donation to maher too.