@asonimie - Problem is you're going to be up against other preforms, so you can expect them to have professional runners who won't be caught by you so easily at midfield. You might do ok against pugs, but that changes entirely when the other team has a strategy too.

Assuming that the other team is a preform of individuals:
WSG - I can't see this one going well normally. While the boxing team will generally be split into 2 five man groups where the other side can split into 2 and 8 or any other combo easily. I don't see this going well unless going heavy heavy defense and spitting a boxer to grab the flag and run like hell.

AB - I think the best that can really be expected here is scraping out 3-2 wins. Again the problem will be quick response and the ability to fight 8v5 at the node the other side is concentrating on. My regular pvp group could make those numbers worse, forcing you to defend 10v5. Also, AB is particularly bad about the problem of splitting your group up from death sending them to different graveyards. 3-2 wins against an average pug or pve guild run. Good pve guild runs or pvp guild runs would dominate 3 boxers though.

EotS - This one could work very very well. It's a lot like AB, 3 nodes wins it (counting the flag as a node), but the response times between nodes are much less. The nodes are very defensible and can funnel and concentrate the attackers. No fiddling with flags in this one either, just stand in the area. I can see taking 3 nodes off the break and then luring the other team into a midfield battle and winning easily. Also, one node can be left relatively alone.

Strand - Defense would be pure awesome. And possibly end up with everyone camping the graveyard. I'm not positive about offense, but it might be a possibility to skip the siege and just use bombs. 15 People with bombs on one side would blast the door straight down and leave a couple bombs left over. This might go very very well. Few problems with getting split up from deaths. Maybe throw one person in a vehicle and put him on cruise control just to distract. I really think this could go well. The other team also wouldn't be expecting a bombing run and would be back on their heels with several in the guns. Concentrated fire on the guns would also eliminate that threat well too.

AV - Concentrated fire in the chokeholds is pure awesome, this one could work very well. Not a big penalty in committing people in groups of 5. This is the best bet.

Isle - Like the other 40 man AV, grouping in 5's is no big penalty. Vehicles are a pain and some are very poorly designed for multiboxing (glaives in particular). The airship would be the key here, I've had great success doing bombing runs with my warlock team. It's easy enough to get all 4 into their courtyard. With 3 multiboxers in there, the other 25 players could hold off anything the other side throws forward.

40 mans would be best and choose your poison; vehicles or towers/bunkers.

Of the 15 man I'd pick them in this order Eye, Strand, AB. Maybe strand first.

I just don't think WSG is viable with 2 boxers against a real team.

It's hard to speculate on a couple of these bg's because EotS, Strand and Isle have never seen competitive bg's.