Well you could have Pen values work against TB as well. Then TB doesn't become so amazing. A max human in BQ power armour has 15 damage reduction (6 for TB and 9 for armour). But if Pen worked against that then a bolter would reduce that 4 and then add 1D10+5 Tearing damage as well.
Perhaps called shots also halve TB on the location being struck? Representative of the weak points being targetted.
Hellebore