No, it will not affect the AP80 performance as it is not related to the internal system. Regarding the size, it depends on your needs in terms of music. We highly recommend buying your MicroSD Card in legit places. Since the AP80 is dedicated for Hi-Res format audio and if you have a lot of songs, we suggest you to use a MicroSD Card with the biggest size (up to 1TB for the AP80) as Hi-res formats use more storage.
No, It is currently not possible to change the battery in the AP80 as it is embedded. Nowadays, all smartphones battery can't be changed. However, the AP80 has a good playtime so you can listen to it for a long time. :)