The core, most important parts, CPU, GPU, screen resolution, are all iPad 2
And it is very likely iPad 2 can do Siri, just Apple will not allow it.
CPUs are the same in the 2, 3, and mini.....the 'X' in A5X simply adds GPU power as evidenced by equal benchmarks for the three devices.
I stand by my point that the mini is just as close to the iPad 3 (in fact maybe closer) than the iPad 2.
Siri is the same as an upgraded camera....a new feature that you pay more for. Just because the software could be run on the iPad 2 doesn't mean the feature isn't a differentiating factor between two devices.....Apple could've added an 8 mp camera to the iPad 2 when it was made....but they didn't.
Just because it is a piece of software, doesn't mean it's not a feature that moves the mini closer to the 3....despite your personal opinions about Siri's usefulness and effectiveness.