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Hi Brett.
The 8.5mm XF is probably as high as you would want to go in power with the ETX 125. You are right, magnification will affect brightness. And you are not that far off in your assessment of the difference between the 17 and 8.5mm. Based on magnification alone the 17mm should be 4 times, ie 400%, brighter than the 8.5mm. ((17/8.5) squared = 4)
Re 12mm XF vs 13mm Hyperion. IMO the XFs are some of the finest EPs available. I do own a 13mm Stratus which is very similar to the Hyperion, and it is a great EP for the money. (Also own 8.5mm XF and used to own 8mm Stratus. Never tried 12mm XF.) IMO, the Stratus is not in the same league as the XF. I'd trade my 13mm Stratus for a 12mm XF in a heartbeat (which btw is similarly priced amazingly).
I'd expect the XF in your scope will be practically edge-to-edge perfection. (the only complaint I've read about the 12mm XF is mild field curvature in fast Newts - unlikely to apply in a slow Mak.) The Hyperion will have more field, but also barrel distortion which gives a "ring of fire" effect (brighter near the edge than in centre) and chromatic aberration outside the central 45-55 degrees. Contrast and transmission is better with the XF. Hyperion wins on comfort because of the rubber eyeguard. But the XF with its adjustable eyeguard is quite comfy also and the light weight of the XF is great too. but as you own both types of EPs you can judge ergonomics issues for yourself.
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