If you are genuinely serious about doing night photography you would be much better off with a full frame camera. A used 5D Mark II (or maybe even a well used 6D if you're willing to wait on a good deal) can be had for about the same price as a new T6i. The 5D Mark II is also the camera for which Magic Lantern was created. It is still perhaps the most mature version of all of the ML implementations.

Might the older model actually perform better?

T2i vs. T5i - Not better, but perhaps as well. The T2i through the T5i have basically the same sensor. It's the same sensor that was introduced in the EOS 7D way back in 2009. Any in-camera improvements to image quality as the models get newer are a result of processing and an improved jpeg engine. These improvements can be more or less replicated using any of the previous models that use the same sensor by saving the files in raw format and applying the improvements in post processing using the most current version of Cannon's Digital Photo Professional.

T2i vs. T6i - Probably not. The sensor in the T6i not only has higher resolution but is also more efficient than the previous 18MP APS-C sensor used by Canon. Even with the smaller pixels it still has slightly better performance regarding noise at high ISO.

What else is going to affect the image quality?

Your choice of lens. Roughly 2/3 of the hardware equation with regard to image quality is the lens. Only about 1/3 is the camera. Apart from the camera and lens: a sturdy tripod or other secure platform for your camera and a way to engage the shutter without touching the camera. For night photography I find a wired cable release works best for me. Some folks prefer an infrared wireless remote.