I haven't shot 300 yds but I did get to use a Leupold Vari X-III while in Africa last year. I can tell you that it had fabulous light transmission in very low light conditions (just before full dark) in the bush.
The best news is that Cabelas and Grafs are running excellent sales on certain Leupolds right now.
Someone earlier mentioned Minox and though I haven't used their rifle scopes, I have a set of their 10x42 binoculars which served me very well in Africa and are great in the Missouri woods.