There will be no performance gains in changing the fluid. Only freshening up what is there. However as it gets older it can become more water contaminated and thus the boiling point will continue to go down. Perhaps in the future you may find that it boils, but seriously, in the conditions you describe I doubt it.
The SS lines are more for pedal feel than anything else. Some makers promote huge gians in stopping distances. This is usually due more to the driver being able to better modulate the pedal, so perhaps under extreme conditions one can argue that the gains are real. (They usually fail to mention the fit of new pads and rotors on some of those tests too)
Lastly, if you are up for the task of installing the lines you have to bleed the system anyhow so a good flush is in order. Then agian if saving the time and $150 is more important....forget about it.
