I love the minis dsp and I think the open drc has more distance setting ability. Even though you may be able to get the OS's equidistant from the MLP the flexibility is nice to have. You won't believe the difference with two properly aligned. More head room, more even response and possibly better decay times.
For the level your getting at I would really like to see you start measruing with full sweeps (10hz-20khz). There is no doubt the second OS will help no matter what but believe me once you can see what is going on you can make a huge difference in sound quality.
You have an awesome set up and you want to get the most from it. At first REW is a bit of a PITA but after you get it working properly and take some measurements its a breeze. I still don't use all the functionality but between FR, decay, and impulse respones you can really take things to another level.
Your system is awesome, even 228's for rears, wow! Very cool. Don't know if you live in the midwest but I will probably have a GTG this fall.
Pre-Pro: Emotiva XMC-1, DSP: Mini DSP 2x4 balanced
Amplifier: Sunfire TGA7400, Lab Gruppen FP10000Q
Speakers: JTR Noesis 212HT (LCR), JTR Slant 8's (surrounds), 3 Orbit Shifter LFU's
Sources: PS3, HTPC, Dish Network
Projector: Epson 8350
Screen: Seymour XD AT 138" diagonal 16:9