How much does power amplifier really matter comparing to other hi-fi components?

25% each is about right. Except you left out wire. Your 25% amp budget is for preamp and amp. I'd go integrated if I were you. Your 25% wire budget has to cover all power cords, interconnects, and speaker cables. Unless you want a really good system and then it should be 20% each, and you add 20% for vibration control and room treatment. Now you're talking!

Everything matters especially amp and preamp.

Depends on the loudspeaker. If you have something really efficient, a nice pair of Quicksilver monos will do the job. But if you have Magnepan 20s, the sky is the limit. 

Matching amps to speakers is a critical consideration. Depending on the 1. CD player / DAC and the 3. Power-amp compatibility;  the 2. Pre-amp might be superfluous.

It’s hard to say. It really depends on the chain

In a recent video series Jay compared a rogue audio class D amp to a high end amp. I dont remember if it was duel mono or not because he didnt specify the elite amp, but for sure it was over $30k vs an amp less than $2k.
While differences were there, they wernt profound and many couldnt even tell the difference

My point is, Jay’s chain is unreal. Hes using power cords that are more expensive than most peoples entire hifi setups. In addition he has 2x Everest power conditioners as well as Synergistics top of the line conditioner. Then add to it the pre amp, the super high end interconnects, his very substantial electric setup behind the wall, ext..
All of this helped to make up for many of the cheap amps inadequacies as well as color the sound and change the sound signature.

If you were to take the same amps in a more modest setup I’m sure you would hear a much bigger difference between the amps

So to sum up a lot of it depends on the rest of the chain. In a mega high end setup the amps impact is much more minimal as long as the power and current capability is there.

In a more modest setup the amp choice will matter much more.

i think the 20-25% figure is fair