I still don't have an answer on the cartridge, but do have opinions on the scopes. How much light gets through is determined by both objective size and magnification. If you divide objective diameter by magnification the number you get is the diameter of the beam of light that comes out the rear of the scope.
A scope with a 40mm objective set on 8X will let exactly the same amount of light (5mm) through as a 50mm scope set on 10X, or a 35mm scope set on 7X or a 20mm scope set on 4X. You can get bigger objectives, and lower magnifications and let more light through, but the average human eye can't use more than 5mm. Some guys still in their 20's with better than average eyesight can use around 6mm. Any more is wasted.
A bigger objective just allows you to use more magnification. Here, and in most places legal shooting time is 30 minutes prior to sunrise, and 30 minutes after sunset. I've made shots on game 5 minutes into legal shooting time with 20mm scopes set on 4X and could see just as well as I could with a 40mm objective.
The 2.5-8X36 used to be my favorite scope, but I'm of the opinion now that I'd rather just have a 3-9X40 for all around use and if I want something smaller I'll skip right over the 2-7X and 2.5-8X and go straight to 1-4X20. The 2.5-8X isn't significantly different in any way compared to a 3-9X40, or 3.5-10X40. You'll never notice the difference on either the low or high end of magnification. Not enough difference in size or weight to matter either and there are just lots more good optics and at better prices in the 3-9X40 class.
Up close 1X is amazingly fast, and 4X is enough to cover 90% of shooting needs. There is enough light transmission to be usable in low light at 4X.
And another vote for Talley mounts.