Has the "It's over 9000!" meme from DBZ spread back into Japan? Is it referenced in other manga or anime? If it is, do they use 9000 (which is, depending on who you ask, a translation error or an intentional mistranslation) or 8000?
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.
The Japanese version of this meme could be considered:
My power level is 530,000
Which is close enough but didn't originate from the same scene. The joke also propagates to any score measurement in any setting. For example one can brag about their TOEIC (Test Of English for International Communications) score:
My TOEIC score is 530,000.
See https://dic.nicovideo.jp/a/530000 for more examples or just google the phrase.
That meme was started from the original DBZ but it got popular after TFS (Team Four Star) made a parody of the episode. And yes, it has mostly spread through Japan. But if you check, Nappa's power level was 4,000 and Vegeta was 2x more powerful, so Vegeta was still surprised. In conclusion, "It's over 9,000!" has spread to Japan.