It doesn't work like that. Stars don't "add health/strength points" to a unit. They instead give the unit certain bonuses.
The game has a file in its data folder called exp.pzdat. That contains the bonuses every unit gets per star of experience (in the new 1.20 patch rules that differs from unit to unit, e.g. infantry gets different bonuses than level bombers.)
An example:
An infantry unit gets the following bonuses per star of experience:
Initiative +0.5
Soft Attack +1
Hard Attack +0.5
Ground Defense +1
Air Defense +1
Close Defense +1
So yes, a 14 star Firefly is stronger than a green Firefly even though they are both strength 10.
however, the unit strength points don't exactly correlate to above stats. They basically represent the number of shots a unit can dish out and the number of shots a unit can absorb.
I don't know the EXACT mechanics but the combat "basically" works like this:
The game determines by initiative who fires first in an engagement (initiative values are modified heavily by terrain and weather and other stuff too, not just experience). Then the first unit fires, the other one takes hits and suppression, then what's left of it fires back. The strength points determine how many 'shots' a unit get to inflict damage on the other one. So basically a 10 strength unit gets 10 shots, a 14strength unit gets 14 shots. The stats like Hard attack and ground defense then determine how much damage potential each shot has and defense determines how much of that damage is absorbed without causing losses.
So.. you see a 4 star unit at 10 strength will already do more damage, or absorb more damage without casualties. if you increase the strength points, its shots will not only hit harder, but now it also got more shots to dish out.
This is already abit simplified, there are loads of modifiers and even random values that are to be taken into account, but those are the very basic principles behind it all.
On top of that it is important who attacks who, infantry vs tank, infantry vs infantry... what terrain... what weather.. is it a ranged attack or a normal one, or from the air....
So saying whether a 7 strength 4 star unit is still stronger than a 10 strength zero star unit cannot be answered. The only valid answer is "it depends".
if that firefly attacks infantry on open ground... maybe, you'd have to check what the combat result screen tells you. but I'd guess at 4 stars it might still get a better result. But... that's just my guess... and only because it might get a better result in that one occasion, that doesn't mean it is in general "stronger". This game takes too many variables into account to be able to say one unit is always stronger than another one. It all depends on "under which circumstances against what enemy"
But my guess is that your real question is: is overstrengh worth it.
Short answer: Hell yes
Long answer: it depends. it is expensive to keep around. if you are short on prestige, I'd recommend not doing it for every unit. most experienced players which play on difficulty levels that force them into prestige shortings prefer to only overstrength units which usually don't take damage during a scenario if played well, i.e. artillery, bombers, maybe fighters (because in fighter vs fighter combat every advantage counts a lot even though you usually take damage), maybe anti air too, and leave the standard fighting units like tanks and infantry at 10 for most scenarios, maybe overstrengthening some infantry and tanks for tough scenarios like big city maps, streets of moscow or stalingrad etc, because those usually end up in heavy losses and overstrength units can sometimes neutralize enemy units before they can shoot back at all, hence preventing losses, but that doesn't always work. but at least it gives you a better start for such scenarios and you can absorb losses a little longer before you have to reinforce, saving you some turns.
Doing that on every map would cost huge amounts of prestige though.
So yes, it is worth it, but you got to have the prestige around to do it.
Another BIG thing to consider is: Basically overstrengthin tanks is most of the time not worth it with the new rules. The reason is: If you upgrade a unit to a better one, it loses all overstrength. E.g. if you upgrade a Pz IVF2 at 15 strength to a Tiger, the tiger will have 10 strength. The only exception to this is if you upgrade along an upgrade path, i.e. Pz IVD to PzIVF or Bf109E to Bf109F. Those are the cheap upgrades because the new unit is just a modified version of your old one. In those cases you keep your overstrength. (If unsure about those paths, the equipment.pzdat file in your data folder has a column that contains the model for each unit. unit sharing that model entry are in the same path.
So you see, it's not worth overstrenghing units which you upgrade very often when you cannot stick to an upgrade path. For panzers it might work as long as you stay in the PzIV or Pz III line... but when you later go to panthers or tigers... you'll lose that stuff. So you'll have to consider that. Infantry is more stable since you rarely upgrade it and usually within a path. for fighters it works as long as you have Bf 109s, stukas work too. Strategic bombers not so much, there's nearly no bombers which share a path. For StuGs it works for a while, but later you leave paths too, towards jagdpanthers etc.
So yeah... "it depends"

Panzer Corps - Dossier Tool - http://www.slitherine.com/forum/viewtopic.php?f=121&t=39151
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