You can definitely handle a lot more NPCs than you think. Make a bigger crowd. NWN2's in game population was a joke... the city was empty and you would down a street and see two people total. If you PC can handle the engine at all, it can handle a good 50 npcs in an area. Shadows seem to be the limitation so make a the obscured people shadows_OFF.
As for 6 player party... it's more about designing entrances (and hallways) with them in mind. For example in my Islander campaign I had a very long but narrow underground temple, where it was mostly water and you walked across narrow bridges. Since there was no real reason not to, I simply put in non-obstrusive triggers that would warp the rear-most party members to that trigger as you walked over it. This was due to the current controlled character + 2 or 3 companions walking fine but the stragglers getting lost and lagging. This ensured the party would reform and be together every 30 yards or so, and most players probably didn't even notice the back of their party warping to the rear of their current PC. But they were definitely less annoyed when they exited the area for some combat, and had the full party in tow.
That's all I had to do really to keep a 6 pack party in control. Wide entrances and the odd placement of those triggers but only in tight areas that have long walks.