Aspd increases work by altering a character's base attack speed. Rengar has a base attack speed of .679. Savagery's 100% (max rank) aspd bonus for 4 seconds means that it averages out to an average 2.7 AD scaling for a total of 4.7 AD assuming Rengar AA's the entire time.

Assuming that Empowered Savagery deals the same %damage as it does now (200%) and given that each point of Ferocity is worth 1/5th of an empowered Savagery and that the new Empowered Savagery grants 3 Ferocity, the new Empowered Savagery now has an effective 3.2 AD ratio.

3.2/4.7 = .68 (rounded down

1 - .68 = .32

In terms of sustained damage, the new Empowered Savagery is now 32% worse than the current Empowered Savagery.

Rengar has however improved is in his burst (surprise!) because of the Savagery chains. If anyone survives his burst, he is worse off than before.

The best case burst scenario (which I'm assuming he's now being balanced around)is where Rengar has all the prep time in the world and knows where you are before he ults. He first activates Q2 then ults and pounces exactly 1.5 seconds later. He chains another Q2 on top of normal Q and in .75 seconds he Q2's again. In this time he hits W and E simultaneously and Q2s again. Finally, 1.5 seconds later, he Q2s again.

He can get off 5 empowered Q's in less than 2.25 seconds + 1 normal Q + E (let's say 1.0 new ratio) for a total of 1200% AD in damage. This is assuming the target doesn't do anything to Rengar in the meantime. If he eschews the last Empowered Q and times his jump so that he gains Ferocity *just* after the second Q, he can theoretically put 1000% AD in less than a second.

However from stealth or if he uses his ult for movement speed, without that Ferocity gen, he'll lose not 2 but 3 Q2s from his combo, doing only 600% AD damage (His current form doing 672% AD in 4 seconds)

In lane, the only chance people have to fight/escape him is by interrupting his Q combo (dashing away, stuns.) However, if you lack the means to kill Rengar in slightly less than 4 seconds, any time you waste a dash/stun while Rengar is at max Ferocity gives him the opportunity to pull off his 4 second death combo.

There's also the fact that Rengar can stealth after the Ferocity has been generated, allowing him to wait for his cooldowns. Assuming Rengar builds tanky enough (for both him and his opponent to survive), he then moves on to Death combo Beta. 3.6-5 seconds (depending on his CDR minimum 17% required) after Death combo Alpha he will tag you with Q and stealth. 5 seconds later, all of his cooldowns will have reset and he will combo you with 2 Q2s and finish you off.