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How would this do competitively?

Librarian - Psyker/Terminator Armor/Storm Shield - 125

5 Terminators - Cyclone ML/Chainfist - 230

10 Marines - Lascannon/ Plasma Gun/Combi-Plasma - 185
10 Marines - Lascannon/ Plasma Gun/Combi-Plasma - 185
10 Marines - Lascannon/ Plasma Gun/Combi-Plasma - 185

5 Bikers - 2x Grav Guns/Combi-Grav/Meltabombs - 150

10 Devastators - 4 Flakk Missile Launchers - 240
Thunderfire Cannon - 100

Aegis Defense Line - Quad Gun - 100

Total: 1500

It seems balanced but there's no "big threat" and you're very vulnerable to any army that concentrates anti-MEQ firepower or doesn't care much about Lascannons. It looks like you've built a gunline army, but you'll probably get rolled over fairly hard by an IG or Tau gunline.

Flakk missiles are VERY expensive, especially at 1500 pts, and you already have an Aegis line - for that price you could get a Stormraven and still have change.
Also, personally I prefer missile launchers over Lascannons in tactical squads, their flexibility just makes them a bit more, uh, tactical!

You really are not making use of the best the fists have to offer. Here is my 1500 point list for comparison.

Imperial Fists 1500 Pts (1495 Pts)

HQ
Pedro Kantor (185)


Troops
Tactical Squad (215 Pts)
10 Models w/ Plasma Cannon, Plasma Gun, Combi-Plasma
Drop Pod

Tactical Squad (215 Pts)
10 Models w/ Plasma Cannon, Plasma Gun, Combi-Plasma
Drop Pod


Elites
Sternguard Veterans (205 Pts)
5 Models w/ 2x Heavy Flamer, 3x Combi-Flamer
Drop Pod

Sternguard Veterans (205 Pts)
5 Models w/ 2x Meltagun, 3x Combi-Melta
Drop Pod


Heavy Support
Devastator Squad (235 Pts)
10 Models w/ 2x Heavy Bolter, 2x Missile Launcher w/ Flakk Missiles
Drop Pod

Devastator Squad (255 Pts)
10 Models w/2x Lascannon, 2x Missile Launcher w/ Flakk Missiles
Drop Pod