I created a mod of web-throwing and flame breathing dogs a while ago, to play around with alternative fortress defenses (no cage traps or weapon traps, just web and fire-breathing dogs).
I found that though the flame-hounds were extremely good at catching and flaming intruders, as soon as you chained them up they were pretty awful at getting the intruders on fire in time.
I found that the following increased flame reaction time and/or increased the number of intruders burned:
1. Have an open path from the fire-breather or webber to the intruder (to keep them safe you can trap that path or just set up a pressure plate that pulls up a bridge to protect your cargo when they're in peril). This ensures that your defenses are active and will engage the enemy.
2. I don't know if you can make "hunting" dragons, but I made all my flamehounds into "hunting" dogs, because it increased their line of sight, thus increasing the time each one could spend engaging the enemy.
3. Always have the enemy walk TOWARDS the flames. Stationing the dogs like this:
X_X
X_X
X_X
where the X's were the dogs behind fortifications and the _'s are the path the gobbos took, didn't work at all, since the dogs didn't have enough time to build up and release the fire.
However, if you build something like this:
X_________X
xxxxxxxxx_X
X_________X
,
where the gobbos had to keep walking down these long windy passage ways to get to the fort entrance, then the dogs have a much better time building up fire and releasing it (especially since the dogs get two shots, once when the gobbos walk towards them and another when they're walking away).
4. I also built a system of drawbridges, two kinds. The first kind, the kind that raises into a wall and crushes stuff, I used to kill the initial invader wave and transform the entrance from a wide caravan friendly entrance into a maze for the gobbos to trek through. The actual corridors the gobbos would walk through I lined with the other kind of drawbridge, the kind that just "trips" stuff when it flips, doesn't crush anything. I used that kind to stall the gobbos, so the dogs had even MORE time. I just set all those bridges on a lever to flip up and down and set the lever on repeat.
(Another stalling measure could be spikes set on repeat).
5. There's something to be said for the stalling abilities of web dogs as well, since the webs would catch invaders in one place, allowing for easy fire-breathing.
6. A lot of stuff is flammable. I had to make sure that all my bridges, chains, mechanisms were magma safe, since dragonfire is about as hot as magma. Invader's clothing burns nicely, and can cause additional burns for a troop of goblins that has to trek over their fallen comrade's burning socks (and corpse). Perhaps you could line your walkway with coal, or line the walk with stonetraps that you've rigged to DROP coal or some other flammable material (graphite I believe burns well), which would allow for one flame breath to catch the entire walkway on fire, roasting an entire troop with one breath.
The system was fairly effective and a similar system in the caverns below, (but only with web-dogs and cage traps), was used to catch a number of FB's.