Bombastic Demo's only real use is for LD, as Recons get maximum CTR with Level 10 bombs. In LD, it's actually pretty good, allowing you to walk through Shivermist and not get Shocked so badly while dishing out huge amounts of damage as a Recon. However, in the Clockworks, Bombastic offers no CTR, so you'd have to get Elite Bomb Focus modules to make up for it.

Full Volcanic Demomen that want to get their Damage Bonus to Very High will need two Elite Boom Modules. Of course, Full Mad only needs one Elite Boom Mod to max everything, and then can use any trinket they want in that second slot. (It's usually going to be a Heart trinket, because HP good. HP very very good.)

As for Mercurial Demo, you'll need Very High CTR UVs on every bomb you use to max everything- but if you can get these, you'll be Super-Bomber. Of course it's excellent for a Lockdown Recon.

Nitro, DBB, VT, and AoA are all excellent- in fact, AoA is next on my list. You may also want to buy the Expansion some time, or at least get it in a trade (since it will be trade-able sometime, thanks Nick for confirming this :D) because, apart from that whole mission being really cool, you get Dark Retribution from the T3 version. And Dark Retribution is a GREAT Polyp killer, and all around good at kiting. (Just lead a mob of Gremlins or Slimes through two DRs placed just right, there's gonna be some crowns and heat on the floor.)

Also, don't discount RSS- I use that all the time in FSC, and I'm pretty sure it can out-damage a DA against Slags if used correctly. Plus, it makes LD a bullet hell for the other team.