It does less damage, but you also have less health to soak said damage with. I feel like the Boards really get % Health damage wrong because people are too concerned with the amount of damage being dealt. It doesn't matter if it is
or
or
. The real question you need to ask is: What is the number of total hits to kill?
8% is 8%. 126 might be less than 160, but 3500 is also less than 4000. Just because the damage per hit is numerically lower does NOT mean that it will be any slower at killing you. Having less Health might make BotRK and Vayne's W deal less damage to you, but it is still the same % of your Health so having more Health after the proc is always better.
Don't think of it as "Now BotRK's 8% proc will only deal 126 damage instead of the 160 it was doing before". That's backwards thinking. You need to be thinking "How far will the 92% remaining Health after the proc get me?".
3500 - 126 = 3374
4000 - 160 = 3840
I would rather have ~3800 Health left to soak the rest of the damage vs. only having ~3300 to get me through the rest of the fight.
Here's hoping the Armor changes balance out.