With a 1.25" mirror diagonal, in theory they should have the same or very similar light paths, BUT, it's the eyepiece holder on top that makes different diagonals vary.. eg: many are between 78-81mm, yet the Tal version is about 71mm, simply because it has a very low profile ep holder and no stop at it's bottom, so the ep can sometimes slip slightly into the diagonal body itself(without interfering with the light path or touching the mirror, unless it's got a very long chrome barrel bit) . Same idea for a std 90 deg prism, except the bigger the prism inside(giving a bigger clear aperture), the longer the light path. My Tak 1.25" prism(27mm clear aperture?) has a light path of 63mm, yet my bigger Tak prism(34mm clear aperture) has a light path of 81mm with it's big 2" ep holder. I could shave a few mm off that with an adpter similar to what Derek has done above(if i was only using 1.25" ep's).
Mmmm. Hell, hope that isn't confusing. Sounded ok in my head