I would like to suggest that the max limit for uptier / downtier should be brought down to .7 BR instead of a whole BR and hear me out.
with every BR you run into the next generation of tanks that are for the most part borderline OP than the last BR line.
eg- 5.7 vs 6.7
a T34-85 facing a Tiger II H - its point and click for the king tiger facing T34 but the T34 cannot do anything frontally to the king tiger. however if the T34-85 is facing a Tiger II P, it stands a chance to pen the turret frontally and have something it can do as well which makes it a bit more fair and fun for the player that is facing uptier.
9.3 vs 10.3 jets
9.3 player is not having any kind of fun this match up nor does he stands much of a chance.
with a full BR difference the lower BR vehicles suffer.
This issue is an attempt by GJ to manage wait times/sales. The dysfunctional high tiers causes a vacuum which pulls vehicles up to fill teams. MM is a balance between fun/frustration for players. This balance is a sales pitch to draw players into premium time and premium vehicles. GJ is a for profit game and I am sure this balance causes a lot of hair pulling on the part of GJ. They don’t want to kill the Golden Goose but money, as in all companies is how they see their game (understandable). Another cause of 1DL and players keeping throw away vehicles in their line ups. Some nations suffer more then others (wide spread between BR vehicles). Example is USA SPAA vehicles. Huge gap between 4.3 and 7.7. This is just an example, other nations also suffer from this gap in a number of vehicle types. Going to a 0.7 BR range would still leave some nations sucking the hind tit. Remember, anything that may reduce profit becomes the least likely to change. Rethink your line ups to compensate for up tier.