Well, Mainstage seems to run just fine on my 2007 iMac (2.0 GHz C2D), but I'm not running a whole lot of stuff at once. And it's got an SSD installed.
I do have a 2015 MacBook Air 13" 1.6 GHz i5, 8GB RAM/256GB SSD and it's a lot more responsive running Mainstage and Logic Pro X, as expected. I haven't seen much lag at all on either, even though Mainstage can get a little buggy sometimes and eat up the CPU on our quad i7 Mac Mini that is located on stage where I work. It runs pretty much the same on my 13" rMBP (Late 2013 Haswell 2.4 GHz i5).
The 2015 Air is a definite upgrade, and the SSD in the 13" benchmarks over 1,000 MB/s. The Pro is better, and a little more solid IMO and the screen is a huge upgrade. But if you just want to save a little cash, the Air will do you well. I'd opt for the 8GB RAM option though with Mainstage/LCPX running. Or just grab the rMBP if you can afford it or go refurb/open box.