+1 to most of what was said about not bothering to spend money on expensive parts. But one thing i would recommend is higher quality tires and brake pads. Good quality road tires with supple (high thread count) sidewalls do have lower rolling resistance than mid and low end tires. Unfortunately, puncture protection also increases rolling resistance. But there is still a measurable difference with better tires.
Better brakes mean you can go faster/safer. With half way decent brakes like you have, better pads will have a bigger impact than changing calipers.
Since tires and brake pads are rapidly wearing items anyway, it's what I always suggest to people for upgrades since you're not replacing something that still has potentially years of good life ahead of it.