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Hi Andrew
the difference between a 50mm and a 63mm diagonal mirror is irrelevant regarding difference in light gathering. It would be such a small loss you would never notice it at all. More relevant is the difference the central obstruction (diagonal) makes to contrast performance. The 50mm gives about 20% central obstruction and the 63mm gives about 25% central obstruction. To work it out yourself, divide the diagonal minor axis size by the main/primary mirror diameter.
So you may ask next, what is the difference between a central obstruction of 20% compared with one of 25%? Theoretically the smaller figure will allow better resolution and contrast, but in practice the difference isn't that great between 20 and 25%. You would notice more a difference comparing 15 or 20% with say 30% or more, like some SCT's have.
You can also get diagonal mirrors of 54mm diameter by the way. It may cost a bit more, but could be safer than 50mm. If the diagonal is too small, you will notice it on the longer focal length and wider field eyepieces, where the true field is too wide for the diagonal mirror to field (or trap all the incoming light rays). The centre of the field will be just as bright, but the outer edges diminish in brightness a bit, when the diagonal is too small.
I have a 12.5" f6 mirror and use a 54mm diagonal which works well. I imagine you would get away with a 54mm too, even considering your mirror is f5 compared with my f6. But your primary mirror has a smaller diameter, so this balances it out pretty much.
Maybe someone else with a GSO 10" f5 could comment on the diagonal size normally used with these, but I suspect they are somewhat oversize from memory.
Regarding wide angle e/p designs, they can put more of a strain on faster optical systems like f5 or less (f4, etc) because the focal plane of these is not! It's curved and not a plane, and so you can benefit from coma correctors like Televue's Paracorr. Of course, you need to keep in mind that the better wide field designs you get, the better they correct for these types of aberations, especially towards the edges of the field of view. You get what you pay for basically. However, you can still enjoy most wide field designs with newtonians of f5, as most people already do.
Hope this helped a bit,
Cheers.
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