Can a digital pot and op-amp challenge the performance of a programmable gain amplifier
What would you be trying to improve? Performance? Cost? Availability? 

There are no guarantees on the noise, the offset drift with temperature, the gain tempco. There is a guarantee on the gain error. 

The typical noise is not bad for a CMOS-input type. The typical gain tempco is excellent (4ppm/K), but no indication of the drift with time.  

Offset voltage drift with temperature (4uV/K typical, no maximum) looks like the biggest error source to me- though if you have a load cell that's zero'd or tared at every measurement like most bathroom scales, it probably doesn't matter much. Same with the initial offset- it's possible to do 50 or 100x better in those departments with a separate op-amp. To do better in the gain tempco department you might have to go to discrete precision resistors and an analog switch, but that would push the price up considerably. 

Sounds like you're really tight for space. Do you really need the PGA or could you just use more bits in the ADC? A (real) 20-bit ADC ("24-bit") would give you a resolution of a few uV in a few V, which is probably at or below the noise floor of that op-amp.