Is the Excitation Voltage of a load cell a strict specification?
Load cells are ratiometric, so the output voltage will be proportional to the excitation voltage.  Operating at a low excitation voltage will reduce the fullscale output from the gage, as you have identified.  This isn't really a problem other than the fact that the differential output voltage of a load cell is so low to start with.  

Assuming that you're using a full bridge load cell (all four resistors in the Wheatstone bridge are actual strain gauges), you can probably expect an output of 3mV/V.  At 2.5V of excitation, you'll have +/- 7.5mV fullscale output.  Even though the output impedance of most load cells is low, noise can still be a problem.  Note that with a 2.5V excitation, you'll be giving up 12dB of SNR.

In your application, I'd stick with the lower excitation voltage.  A boost converter will add noise that may be difficult to reject with your sigma-delta ADC.  

(As an aside, exceeding the maximum load cell excitation isn't recommended as it can cause self-heating and resultant errors due to inconsistent changes in resistance.)