I like lvl 15 armories for temp buildings. They're 4x4, fairly cheap and fast to build. I put them out in guest race spaces between chapters to drive up training size, then knock them down when I need the space. It's not like troops decay, lol, so I make more of them at times.
Lvl 1 Shrooms is 6x armory levels for training size boost, so each one adds 40 for its own contribution and 90 for Shrooms boost, so a total of 130 extra training size per armory.
Where I really see a huge difference is in my Ch 15 city with a lvl 18 Shrooms that gives 26x number of armory levels: When I place 5 lvl 15 armories, I get +600 training size from them and +1950 from their boost to Shrooms for a total of +2550 training size. It's also helpful because if training times get over 3hrs or so, I can just kill a small armory. But when you're making more squads/slot, longer training times aren't as limiting as I originally thought they'd be. My Needles there is lvl 21, too, so that helps training times for barracks units.