Upon knowing that there's a 25mm rear port opening for a C5, 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 25mm. From my test target, I could see slightly more than 3 squares plus a tiny bit of space with a 1.25" but I could see nearly 5 squares with a 2".