I don't want to be a wet blanket here, but I think my experience is worth sharing. My first dungeon build was designed around alchemy and the elixir gun - I wanted to be a healer/buffer. Like Toaster, I got hammered every time I went in an instance, and I felt like I wasn't contributing enough. The movement requirements of some fights are such that I couldn't get my group to stand still for my super elixir or healing turret, so all those HoTs were just wasted. Next, I tried a tanky Flamethrower build, and the first dungeon we did was Ascalonian Catacombs. We tried one of the paths with all the graveling burrows, and knowing that those things had to die FAST, I planted myself right next to them, and MissMissMissMissMissMiss. I had to go respec so we could finish the instance. In sum, I've tried the FT/EG setup in dungeons, and each one separately, and it hasn't gone well. That's partly the fault of buggy mechanics with the FT and in some of the dungeons (the burrow hitboxes are actually underground, for example).
So if you have a hard time with FT/EG, don't beat yourself up. I've sort of come to the conclusion that these types of builds are very situationally useful. If you haven't got one of the preferred situations, don't be surprised if you have some trouble. It might be worth having a rifle/grenades build + appropriate gear, just in case. I keep going back to my 30/20/0/0/20 build, particularly for dungeons. The FT is great for the Orr DE circuit, world exploration, and WvW, but I've had a lot of trouble with it in dungeons, and I'm grateful to have so many backup options as an Engineer.
I am thinking of sticking with his build but instead of Knight's armour I am thinking of trying out a Dungeon's Power/Toughness/Vitality armour set so I don't die as often. +300 precision might be more than enough already, so rather than that additional precision, Vitality might prove more useful. Maybe someone that has more time to play and sucks less than I can let us know how it works out with Dungeon armour over Knight's.