8 won't stabilize a 90gr SMK either, it you hunt a 70TSX matters but most don't use either. Everyone must make their choice based on the bullets they plan to use, I wouldn't buy a 7 twist if I'm shooting 38gr varmint grenades.
The Barnes 70 grain TSX is shorter than the Hornady 70 grain GMX.
The 10-shot group pictured below was fired at 100 yards from a semi-automatic AR-15 that has a barrel with a 1:8" twist using Hornady 5.56 TAP 70 grain GMX The group has an extreme spread of 1.16".
The Barnes 70 grain TSX is definitely unstable when fired from a 1:9" twist barrel. The group shown below was fired from an AR-15 with a 1:9" twist barrel. Accuracy was terrible and there were definite signs of key-holing.
Depending upon what reference you use, the Barnes 70 grain TSX is stable, to marginally stable in a 1:8" twist barrel. I've never had any issues shooting the 70 grain TSX from barrels with a 1:7.7" twist. The 10-shot group pictured below was fired from a semi-automatic AR-15 with a Krieger barrel with a 1:7.7" using a hand-load of the 70 grain TSX. The group has an extreme spread of 1.12".