I'm still not seeing any benefit of using the new software without the new microphone. If I did my measurements again with the new plugin, would they be more accurate? Have there been improvements to the EQ? 

My previous job was an applications engineer for the largest MEMS microphone manufacturer in the world and compared to condensers I can tell you MEMS microphones are:
 
1) more stable over time
2) more stable over temperature
3) more stable over humidity changes
4) part-part variation is very low 200-10kHz
5) more noisy
6) more expensive (generally)
 
So, I'd imagine you can be more confident that the system will work better (due to #4 above) over a longer time period (due to #1-3 above). I also imagine the advertised +/- 0.5dB accuracy is only when the new MEMS microphone is used. Condensers at this price point can't hold to part-part tolerances this tight. Even if IK screened mics while building the older ARC2.0, they won't hold to +/-0.5dB over their lifetime. MEMS mics will.

BobF
From  http://www.ikmultimedia.com/news/?item_id=11087
 
 
"The new ultra-accurate MEMS measurement microphone works together with the room correction software to achieve greater correction accuracy than ever before with a precision of +/- 0.5 dB, (previous ARC microphones will also be supported.) IK is the leader in MEMS technology for audio applications and ..."

cool, that's the info i needed, thanks