Upon knowing that there's a 37mm light baffle inside the drawtube of my Borg 45ED II, I tried to see if a 2" eyepiece can really give wider field of view than a 1.25" one.
I tried a 20mm Widescan Type III which can maximize the field stop of any 1.25" focuser, i.e. around 27mm, and a 40mm Pentax SMC XL which can maximize the field stop of any 2" focuser, i.e. around 46mm.
Experiment shows that the 2" gives far true wider field of view, that means at least a 2" diagonal is not going to be a complete waste, despite it's just 37mm.
Also, I found that my Widescan cannot reach focus with a 2" diagonal, but a 1.25" diagonal works.