better bars = meh, not that different
better brake = meh, most brake work well enough
better wheels = even though DT axis 3.0 is better, it is not that much of an upgrade.

Spend your 300$ on after market wheel upgrade. Or even better, work out a deal with the LBS to upgrade to Ksyriums or something.

I am cheaping out with the tarmac mid-compact with sora, lol.