Even with that scenario, you could still keep a cannon escort build while moving out to snipe with your Javelin. That would make for some interesting strategies involving speed and movement buffs, coordinated repositioning, and the usual offensive buffs.
I like the 15Km range but don’t forget that the javelin needs to power up thru weapon hits. So you would need to extend the range of other weapons to really make this viable and I don't see that happening.
That said the other suggestions look good:
1) Increase the turn rate while in siege mode.
2) Increase the shield regeneration rate while in siege mode.
3) Increase the damage of the javelin.
4) Allow all weapons to fire when the javelin is firing.
Here are my own suggestions.
1) Give the javelin a 30% shield penetration ability that will do direct damage to the hull.
2) Give the ship a 15% resistant buff to both energy and kinetic damage when in siege mode.
3) Remove the ridiculously long delay in firing the javelin.