I've finally heard all five m@ster version songs for the Namassuka Specials. Here is my personal opinion on their ranking from most improved to least improved (or made worse, in one case). This is not related to how much I like the song.
Honey Heartbeat > Nanairo Button > Ai Like Hamburger > Little Match Girl > Kyun! Vampire Girl
Honey Hearbeat: New lyrics, new singing, multiple bridges, a guitar solo. Just overall a highly improved product.
Ai Like Hamburger: I think the new parts are really well done, and I also think that Yayoi and Ritsuko sound really good together in this song.
Nanairo Button: I really like the bridge and I think it sounds awesome. That's all that was added, however.
Little Match Girl: This song already had a m@ster version that was performed at a concert, and I'm a little bit disappointed that they didn't update it in the past year.
In terms of changes, the only thing they really added was a bridge and some more lyrics. Chihaya has my favourite version in-game and I think I might like that version a little bit more. Overall, I feel that this one is about the same or slightly greater than the in-game version.
Kyun! Vampire Girl: I like Miki's singing in-game more, I think the really short bridge sounds pretty bad, not to mention that it's only really sung by Takane, and nothing else was added aside from more lyrics. Quite disappointing overall, in my opinion.