Interesting question. Based on what I've seen from fighting against Sinesha in arena, I think that the 'level' is a percentage, not an absolute number. I've certainly never seen champions suddenly appearing to have more HP points than their normal max HP. So when she does the equalize and the least damaged champion is still at e.g. 90% health, everybody else will get back to 90% health as well, which perhaps for one champ will mean a 5k heal and for another a 30k heal. As far as I know, though not quite sure, 'the ally with the highest HP' is also selected based on percentage of total HP remaining, not based on who has the highest absolute amount of current HP. 

So there's no need for any cap and the boost doesn't go away. 

Most people already do what your saying.

They do this philosphy with heroes who do Ally Protection.

Ally Protection redirects incoming damage.

For Example:

Jareg does Ally Protection.

Ally Protection is a buff which goes on 3 teammates excluding Jareg.

When the 3 teammates get attacked, They don't take the full damage.

Part of the damage is redirected to Jareg.

Ideally, you want the heroes who have the Ally Protection buff to be high DEF.

High DEF allows them to mitigate the incoming damage

Let me give you some numbers to help make sense of the situation.

Lets say the enemy is attacking you with 1,000 Damage

Lets say you have 0 Defense

Your hero will lose 1,000 Health because you have no Defense to Mitigate the incoming damage.

Lets say the enemy is attacking you with 1,000 Damage

Lets say you have 2k Defense which Mitigates 50% of the incoming damage.

Your hero will lose 500 Health because you have 50% Mitigation

Lets say the enemy is attacking you with 1,000 Damage

Lets say you have 2k Defense which Mitigates 50% of the incoming damage.

Lets say you have Ally Protection Buff on your hero

Your hero should lose 500 Health because you only have 50% Mitigation 

However, the Ally Protection buff will split the damage among 2 different heroes.

The Ally Protection Sender (Jareg) will lose 250 Health.

The Ally Protection Receiver will lose 250 Health.

For this reason, Players like to run 1 high HP hero with 2 or 3 high DEF hero's.

The perfect situation would be 1 high HP + 3 high DEF.

However, Players have tried other set ups which have worked.

They have done 1 high HP + 1 high ATK + 2 high DEF

They try to squeeze in a Squishy Attack type Damage Dealer who can do massive damage.

Another set up which people have tried.

They have done 2 high HP + 2 high DEF

As for the other question about Sinesha.

I will answer it in this comment vs. other one so I don't confuse you.

Lets say you have 3 heroes.

Jareg has 100k Total HP, but is currently at 35k HP

Sinesha has 50k Total HP, but is currently at 20k HP

Skullcrown has 30k Total HP, but is currently at 10k HP

You use Sinesha A3 move to Equalize.

It will take the champion with the highest HP at the time.

In this case, the hero with highest HP is Jareg.

Jareg is sitting with 35k HP.

Jareg Total HP is 100k, but it isn't what he has at the moment of you using Sinesha skill.

So what happens to the other heroes?

Sinesha has 50k Total HP + Currently is at 20k HP

After the move, Sinesha Current HP will change to 35k HP

35k HP out of 50K Total HP

Skullcrown has 30k Total HP + Currently is at 10k HP

After the move, Skullcrown Current HP will change to 30k HP

30k HP out of 30k Total HP

Skullcrown can't reach 35k HP because 30k is her limit.

Skullcrown will simply get a full heal in this situation.

Interesting Player J, actually i didn't think that was how it worked, but your explanation makes sense.  For some reason, I thought that it would (using your example) bring up the max HP of all heroes to the one with the highest HP.  

That would be pretty way too OP now that I think about it.