I'll assume Aurelion is max level/ranks in W with 800 AP, no MR pen.
Sivir has 1910 base health at level 18.
Most ADCs don't build that much health except for things like Sterak's occasionally, but are more leaned towards offensive+resist hybrids (Maw/etc).
GA being the exception, for just pure survivability in late.
Sivir with base resistances + runes = 42.06 MR
That's a 0.703927918 multiplier.
1845 for 3 stars now becomes 1298.74701.
Still a large amount.
With Maw of Malmortius as well = 0.549269472 multiplier.
The 1298.74701 is now 1013.40218.
With GA as well = 0.440431623 multiplier.
The 1013.40218 becomes 812.596344.
Sure, it helps with that much MR.
The issue is, with that much MR, Sivir (being a non-AD caster type ADC, like most ADCs, AD casters being like Jhin for example) will lose a VERY significant amount of damage, as she relies heavily on crit and attack speed over just plain AD.
She only has 4 item slots left, 3 if you count boots, 2 if you count a LW evolution.
Even with full crit/heaviest stat items (like Phantom Dancer) for example, she will only achieve 60% crit, and that means forgoing Infinity Edge/Essence Reaver as well.
I dunno. I'm probably just bad and dunno what I'm talking about.
