I love T8 ships, and I really want to defend them from being up-tiered all the time, but to be honest, T8 ships have just as much of an advantage over T6 and T7 as T10s do over them, if not more. It's quite easy to like and feel T8s are awesome and fun ships when they have complete mindblowing advantages over the tiers lower than them. When I analyze how they outperform Tiers 7 and 6 I do think it's fair they're not preferrably top tier.
- T8 Battleships have 32mm plating, they can angle to ignore most or all forms of AP damage from T7/6 ships.
- T8 Battleships have more than 380mm guns, they can overmatch all 25mm armored bows or T7 battleships, doing to them the same Yamato does to T8s.
- T8 Battleships get access to an additional Concealment bonus through the T8 slot, meaning most of them outstealth lower tier battleships and sometimes by hugely significant amounts.
- T8 Cruisers get 25mm armor, being higher than the overmatch value for some T6s and able to bowtank bottom tier BBs. Some of the T8 cruisers have 27mm platings that can bounce 15inch guns which are very common on T6 and T7.
- T8 Cruisers get access to the Concealment module, often outspotting most cruisers that are T6 and T7 again by a significant amount.
- T8 US, UK and Soviet Cruisers have access to Radar, which is already strong against T8/9/10 DDs, is even more effective against T6/7 DDs.
- T8 DDs have access to the concealment module, making them completely outstealth lower tiers (save for a couple rare exceptions). That's vital for cap contesting and also allowing you to permanently spot most T6 and T7 enemy DDs with zero risk of punishment from enemy DDs.
A T8 is in fact a much bigger threat to a T6 than a T10 to a T8 . It pains me to say this, but it's absolutely fair that they're not as frequently top tier. At least your 32mm plating won't be overmatched by Montana, Republique, Kurfurst -- Yamato is the outlier. The stealth across DDs is very consistent between T8 and T10 usually having very small or nonexistent gap between most with all of them having access to Stealth Module. T8 Cruisers are in a bad spot since the Cruiser heal is exclusive to T9/10, so they're probably the only ones being unfairly punished right now by that Matchmaking.
Meanwhile you can't really angle at all on a T6 BB vs a T8 BB. You can't try to spot the enemy T8 DD on your T6 DD as he can have more than a full km detection less than you. These are far bigger problems than the T10s having a bit more RoF through the RoF module, some more range or some more HP than you.
As long as T8 have such a whooping advantage over T6, I can't defend them getting any kind of favourable matchmaking, you'll just transfer the pains to someone else and in fact transfer them to someone who will be hurting even worse!