How can we mathematically conceptualize this?

Player A and creature B both roll for initiative. After a few rounds, player A will kill creature B.

- A wins initiative = A B A B A B A

- B wins initiative = B A B A B A B A

If it takes 4 attacks to kill B, going first means A will defend 3 times instead of 4. With no bonus, A will defend 3.5 times on average, at best that's 3/3.5 = 0.86... something like +14%

If you do the 20x20 grid for both initiative rolls, each will go first 190/400. With a +5 bonus, one will have 190+20+19+18+17+16/400 of going first while the other will have 190-19-18-17-16-15/400. Or 280 vs 105, or 3.27 defenses.

Equation: (105/(105+280) + 3) / 3.5 = 0.935

That's about +6.5% bonus. {close enough to mellored back-of-the-envelope calculations, nice}

That also means that a S&B Dex20 fighter will do about 106.5% of the damage of a S&B Str20/Dex10 fighter. Melee wasting a turn to reach the target has a much bigger impact, something around +25%.

I don't think it's worth considering.