It's Over 9000!

Quoting wikipedia

In the original Japanese and English-translated mangas as well as the original Japanese anime series, Goku's power level is actually read as "over 8000!" (8000以上だ…! Hassen ijō da…!)."

In most versions of the scene dubbed in other languages, Vegeta actually says "It's over 8000!". However, even in the Remastered Uncut Funimation English dub, Vegeta still says "It's over 9000!", now performed by Christopher Sabat. This change was made due to inconsistencies with the dubbing process; the production team claimed that "9000" fit the mouth flaps of the animation better.

So it's originally 8000, but it was dubbed as 9000 to fit the mouth flaps.

As far as I can tell, it hasn't really spread back very much. However, my method of checking is a bit strange, so it's likely that a native Japanese speaker could answer this better. As you pointed out, the original version has 8000 rather than 9000. What Vegeta says in Japanese is "八千以上だ" which literally translates to "It's over 8000."

I looked on nico nico for any videos with "八千以上だ," but I didn't get any results. Removing "だ" gives some results, but none of them are related to DBZ as far as I can tell. I then tried all variations I could think of, like using "8千," "8000," writing 以上 in hiragana, etc. Still no DBZ videos. Using different versions of 9000 also doesn't get many results. A couple of videos that came up made some references to "9000千以上だ" in the description in such a way that they could be referencing the meme, but the number of these cases was pretty small.

I do get a reasonable number of results by searching "over 9000" on nico. All of these seem to be referencing the English version directly, i.e. they say "It's over 9000" rather than the Japanese version. So it does seem that there is some back-propagation of the meme, but they prefer to keep it in English. Also judging by the number of videos and the number of views on the videos, it seems it never became as popular as it did overseas. That probably shouldn't be surprising, given that DBZ ran from 1989-1996 in Japan and the meme only started in 2006, and mostly spread on English-language websites like 4chan and YTMND.