TLDR: Use at least a 9px font unless you want negative reviews.
This is 美咲フォント, isn't it? Actually, it's indeed 7px per glyph plus 1px padding :) I know this font, but didn't mention it in my previous answer, because I thought you will never need this in smartphone games with LCDs with >200 dpi.
8px (7px + 1px) kanji fonts are used in some real games on 3DS, which is equipped with very low-res screens. For example, the 3DS version of Dragon Quest X and games from Etrian Odyssey franchise use this font (or something similar), because they needed to display many characters simultaneously.
However, such fonts are only readable and understandable with the aid of the context. And even with the context, some kanji are still hard to recognize even to a native speaker. I remember great many Dragon Quest users complained about the difficulty in recognizing characters when the 3DS version was released. Even the native speakers could not distinguish between 鉄の剣, 鋼の剣 and 銅の剣 ("iron, steel and copper sword", respectively).
If you do need this tiny font, you have to be more careful than usual. You have to tell your translators to avoid using complicated kanji in the translation process. Try to use it as sparingly as possible, and never use it in the critical parts of your game.
Example (screenshot from Etrian Odyssey taken from this blog article):
The list on the left uses a 9px (in actual glyph height) font, which I think is not tough to read. Under "SKILL" and "ITEM", there are 7px characters, and I can barely recognize the kanji under "ITEM" (魔神の礎). The 9px font is used throughout the game with almost no problem.
If you are developing a smartphone game and want to add the pixel-art feelings to it, perhaps 10–11px bitmap font is a reasonable choice, which is readable enough, and "jaggy" enough. Or you can use TrueType fonts designed exactly for this purpose (for example, this).