Ikruti, your math is still wrong.
The stacks don't benefit from the AP ratio.
If I have the 5th initiating jump on the enemy:
Old: 5*120+0.5 AP
New: 5*80+0.8 AP
So you need that 0.3 AP ratio to account for 200 damage, or in other words, 666 AP.
Konfetarius, I had a more or less full build non-RoA Kassadin game this week. 99 AP from AA's passive, 45 from it normally, 70 from Void Staff, that's rounded up to 210 with the bit from masteries/ignite (I take the AD masteries in T1).
Still not equivalent.
And this is only equal/better for the first scenario, where you join the fight without stacks. The more ults you get in a fight, or the more stacks you have before you join, and the more of a nerf it is.
And if you built pure AP Glassadin, then still, it's a buff, so long you don't get into a fight with many stacks/use the ult numerous times in the fight. And if you built glassadin, you don't really have the mana to do that anyway.