Adding a TC-1.4 II OR a D7100 for outdoor sports
Assuming that you shoot the 300 f2.8 VR at one stop wider aperture and one stop lower ISO (for equal DOF) than you would the D700 with TC-14E:
1. SNR will be better by about 1 dB. Not much improvement here.
2. DR will be better by at least 1.1 stops. Significant improvement.
3. Autofocus will be faster.