RPBC,
I was trying to do the same thing as you with a Mac Mini. My hope was that I could get the Mini to startup and autoload MainStage 3. Then the P6 could read the patch names and behave just like a hardware synth. I was using the Mac Mini in a rack and the P6 as my only controller. (I was trying to get the look of Jonah Nillsen of Dirty Loops). I wanted to be more a presence on stage and not buried behind keyboards. Nice idea, but it didn't work perfectly for several reasons.
Unfortunately, I was never able to get the program to load correctly. I would get a blank screen that I could easily close, but it required a screen, a mouse or a keyboard to do it. Then I had some problems that I couldn't figure out without connecting a monitor. So... I had a Mac Mini, a monitor, a keyboard and a mouse. So I figured carrying a Macbook Pro might make more sense... which also won't crash if your power goes out. I had $750 in the Mac Mini (updated 4GB RAM + 256GB SSD), a $100 monitor, $100 in wireless keyboard and mouse. It had to travel in a rack ($75) and a rack shelf ($50) and a powerstrip ($35) and a UPS ($50). In the end, I spent over $1,000 to be "portable". I found a current model quad core i7 with 8GB RAM and a 256GB SSD for $1,500 on Craig's List. It runs faster and better than the Mac Mini.
As far as the display on the Roland controller. No, unfortunately you can't show patch names. The Roland is a very standard controller. The P6 and the new Akai Advance series lock up with the software a lot better. However, I gave up on the P6... just too buggy. The Akai sounds interesting, but you would have to use their software... that rules out MainStage. I picked the Roland because I have always liked their build quality. The A800 is a lightweight piece of plastic. However, I can play the keys without hurting my hand (I play a lot of organ that requires me to slide around on the keys). The P6 had sharp edges. The sliders, pads and buttons work more smoothly and feel like they have a better resolution than the P6. The drum pads on the P6 had a slight delay when playing samples... the A800 does not. I built a template in MS3 for the A800 and it has all of the functionality of the P6, even the program selection style.
I've tried most of the controllers out there at our local Guitar Center, I even tried an Arturia 88. In the end, for me, the Roland A800 just feels better. Warning, the A800 has shorter keys that a standard piano. I play piano on an 88 note controller (Roland A88). If I wanted to play piano on the A800, it would be a problem. So, now I have the A800 and the A88, a MBP, a Radial USB interface. It runs very solidly and if something goes wrong (and it always does) I can put my hands on the computer. I still carry a Nord Electro as a backup keyboard in case of catastrophic failure.
Take a look at Billy Joel's current keyboard rig on YouTube. David Rosenthal talks about carrying four MBPs, just in case! I thought about carrying the Mini as a backup, but maintaining two parallel computers would be too much work for me.
Good luck!
Matt