imo it mostly depends on the tanks you play and whether you platoon or not.
i'm playing only shitty tanks atm for grinding ( stock ferdi, stock at15, stock emil, amx m4, stock m53 ) and i get to 58% solo overall while having in most of my t10/9 i play meanwhile 70%+ winrate.
tanks which are depending too much on the map, have bad depression for example and no good turret or are forced to let the enemy run into you are pretty much leading you to losing a lot more games because you can't influence
how the battle develops or simply because can't perform good under certain circumstances. platooning can help a lot if the other ones aren't playing shit tanks, but that only affects winrate most of the time ;D
edit: i is hardly broke on kreditz that's why i can't into gud tenks and am forced to suffer
edit2: i can agree with veit that t9 or really good t8 feel the best way to get a high winrate.