Voltage Reference Output Impedance
The output impedance isn't the series 400 ohm feeding the device, it is the device in parallel with the series resistor. The device is likely to have an output impedance below 1 ohm. Having said that, at high frequencies it'll get worse so I would recommend placing a 100nF cap in parallel to ensure at HF the net output impedance seen by your IA remains low across all frequencies of interest.

The data sheet tells you that the dynamic impedance is typically 0.15 ohms but you should still put a capacitor in parallel because the impedance will rise for significantly higher frequencies. 

EDIT 

Here's a screen shot of what I mean: -



Above 1MHz the output impedance is about 10 ohm so it's pretty good but what happens at 10MHz+ is just guesswork. 100nF has an impedance of 1.6 ohms at 1MHz and obviously gets lower as frequency rises. A slight change of mind would be to use 1uF because then the composite impedance of device and capacitor should never really rise above 1 ohm for any frequency up to well over 10MHz.

Remember, that it's not just "your" signal that you might be fighting common mode problems with but, broadband noise at your input.