OK, it might be a bit weird to see such hight numbers in lets say slowdown or knockdown chances.

BUT. There is a reason behind such values.

First of all every such debuff is affected by resistances, every mob gets 1% of resistance for EVERY level and if its a BOSS monster/man it gets another 50% just for being a badass boss, you know.

And another VERY important thing - resistances are not subtracted from your chance to apply an effect. Your chance is actually multiplied by (100%-resist%).

What does it mean is that if you are fighting lets say level 30 mob with 100% slowdown then the actual slowdown will be 100%*(1-0.3) = 70%. But lets say its a level 70 monster and you are still at 100% slowdown then the actual slowdown will be 100%*(1-0.7)=30% and for level 100 you will be slowing the mob by 0%.

So capping the slowdown(or any other debuff) at any value will make it useless in the endgame. The thing that we need is actually a revert of what we had in the original mod.

So in original mod we have the following formula and behaviours:
Min slowdown cap = 10%
Max slowdown cap = 90%
Scaling = 25% + 50% (1st 100% SI) + 33% (2nd 100% SI) + 25% (3rd 100% SI)

Lets put it onto some keystones.
Keystones are as follows (and we assume that we are playing dedicated sign build where we have ~100% SI at lvl 10ish, ~200% SI around lvl 20, ~300% around lvl 30 and it is possible to get 400% at 40ish, after that you have no way of increasing your SI):

LVL 10 monster - 75% * (1-0.1) = 67,5%
LVL 10 boss - 75% * (1-0.6) = 30%
LVL 20 monster - 108% * (1-0.2) = 86,4%
LVL 20 boss - 108% * (1-0.7) = 32,4%
LVL 30 monster - 133% * (1-0.3) = 93,1%(90%) due to cap
LVL 30 boss - 133% * (1-0.8) = 26,6%
LVL 50 monster - 133%(we are already capped due to original formula) * (1-0.5) = 66,5%
LVL 50 boss - 133% * (1-1) = 0%(10%) due to min cap. From that point bosses will be slowed by 10% every time (exept if you have phetri philtre, which sets your slowdown to 50% min cap ezpz)
LVL 70 monster - 133% * (1-0.7) = 39,9%
LVL 70 boss - 133% * (1-1.2) = -26,6%(10%) due to min cap.
LVL 100 monster 133% * (1-1) = 0%(10%) due to min cap.
LVL 100 boss 133% * (1-1.5) = -66,5%(10%) due to min cap.

So no we got some real shit. That shows us once again why W3 was not supposed to have NGP mode or that uncapped level. It just disbalances things out. What we can do now?

Actually I see the only way to fight this which consists of 2 parts.
First make mobs to gain 0.5% resist per level instead of 1% and bosses base to be 25% instead of 50%. In this case lvl 100 enemy will have only 50% resist allowing sign dedicated players to remain effective without phetri philtre while high level boss will still be barely slowed by.

Second revert the scaling to be "more effect with more power".
And reduce the max cap to ~70%.
Scaling = 10% + 20% (1st 100% SI) + 25% (2nd 100% SI) + 25% (3rd 100% SI) + 33% (4th 100% SI)
Two of theese changes will switch the percentages to this amounts:

LVL 10 monster - 30% * (1-0.05) = 28,5%
LVL 10 boss - 30% * (1-0.3) = 21%
LVL 20 monster - 55% * (1-0.1) = 49,5%
LVL 20 boss - 55% * (1-0.35) = 35,75%
LVL 30 monster - 80% * (1-0.15) = 68%
LVL 30 boss - 80% * (1-0.4) = 48%
LVL 50 monster - 113% * (1-0.25) = 84,75% (70% due to cap)
LVL 50 boss - 113% * (1-0.5) = 56,5%
LVL 70 monster - 113% * (1-0.35) = 73,45% (70% due to cap)
LVL 70 boss - 113% * (1-0.6) = 45,2%
LVL 100 monster 113% * (1-0.5) = 56,5%
LVL 100 boss 113% * (1-0.75) = 28,25%

ALSO the very important thing that this will affect is that Aard and Yrden will be able to do a decent amount of damage to level 50+ bosses and monsters. Coz before every lvl 50 boss was completely immune to EVERY sign exept for Igni.