I'm not convinced that Sonic's universe is 5-D. Containing 4-D structures within itself doesn't automatically make it 5-D, it's just 4-D on a greater scale. I strongly disagree on that part, at least.
As for the other stuff, I actually did plan to bring up the Eggmanland that "transcends dimensions" at some point, though I'm concerned about Ultima's revisions potentially introducing some 1-A tomfoolery into this (which would either make it 1-A or just nuke it entirely). There's a case to be made for it still being 5-D though, given that the English version specifies "this dimension," with Sonic's universe being 4-D.
Maginaryworld is interesting, involving 4 spatial dimensions and 1 temporal dimension if I'm understanding this correctly. In addition, the End of Time brought up in the blog also interests me because it's a realm that continues to exist despite the Time Eater erasing space and time on a multiversal scale, which definitely helps when it comes to arguing it to be a higher dimension.
So... I think I'm in agreement with 5-D at the moment, though I'm iffy on anything higher.