The 60Da is explicitly designed as a dual-mode camera. It can work for regular "terrestrial" photography, or astrophotography. As such, it's increased sensitivity to H-alpha is not that much improved over a stock Canon DSLR, and significantly lower than a full astro-modded Canon DSLR (which has the IR filter removed entirely and usually replaced with a square IR/UV cutoff filter.)
You should get better Ha exposure depth with a 60Da than an unmodded camera, but the changes in that camera's design by no means make it unusable for daytime photography.
The new Nikon D810a, on the other hand, was explicitly designed as an astrophotography camera. It is not recommended for use as a daytime photography camera. It's price is so prohibitive that it's not even recommended as an astro camera, as for hardly any more (and in some cases possibly even less) you can get a proper cooled mono astro CCD camera with filter wheel, and get significantly better exposures than any DSLR.