I look at as if I am setting up an encounter for two parties.
As an example the 4 person part gets the encounter in the book. The 2 or 3 man party gets an CR of (whatever is in the book-1).
Then combine them into one encounter. If map space is limited then use the same amount of XP to make the encounter, but be sure to not use 2 few monsters. Action economy might lead to some easy fights for the players.
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I've been trying to do this fairly in my jade regent game, we've recently added a 7th player so the +1 CR rule isn't quite going to be enough when all players are there. What I've come up with is redesigning the encounters so that a each person in a 7 player party gets enough XP that its equal to a 4 player party as presented in the book (since the AP's are written for 4 player parties). Some of the recalc's Ive done make it out to addince about 1 1/2 CR to the encounter. And really you need to add more monsters not just bump up the monster stats. Wraithstrike is correct on action economy.
It takes work to manage the larger groups.
I would say that an eight member party is APL+2, yes. That seems the mathematically soundest approach to take.
If you take a four person party and start with a CR = APL encounter, then you double the number of monsters, that increases the encounter's CR by 2. If you then double the number of PCs as well, then you're more or less back where you started as far as ratio of monsters to PCs, and thus the encounter should be about the same difficulty overall. So the eight person party would seem to be best represented as APL+2.