Of course adding anyone as a reinforcement would help the kages, but the Gates power is not exponential. That is not how they work at all.
According to the manga, a ninja normally uses 20% of their total chakra at an given time. That is what the chakra points are for in the chakra pathways. (this is what the Hyuuga hit with Gentle Fist to block chakra)
Anyway, there are 8 Gates, and each Gate allows for 10% more of the user's total chakra to be used at once, starting at the base 20% and moving up to 100%.
There are a few side-affects of this, like increased healing when first opening the Gates, increased blood flow, etc. but those don't really do much.
Where most of the power comes from is the user augmenting their speed and strength with chakra. Every ninja does it. Some do it for long periods of time, such as Naruto, Bee, and A.
Thus the more Gates that are opened, the more chakra can be used at once, thus making the user faster and stronger by augmenting those traits with even more chakra.
Follow me so far?
Since each Gate allows for more chakra to be used at once though, the time a user can stay in the Gates decreases with every Gate they open. I would assume it would be 1/8th of the time less for every Gate that opens, as there are only 8 Gates, but I guess it depends on what the user does while in the Gates. They could just stand there and not exert any chakra and theoretically stay in the 8 Gates forever.
That being said, Guy is thus limited by his chakra. Unlike A, Naruto, and Bee, all of whom have bijuu level chakras normally, with Bee and Naruto also having an actual bijuu inside of them to add to that, it is literally impossible for Guy to go as fast as them while in the Gates.
Do you see why I say people overrate the Gates?
First off, people only use 20% of their
muscles at any given time.
Now I am going by your logic of adding 10% more chakra each gate opened.
One thing that you aren't taking into consideration is that the 8th gate is more powerful than all of the other gates and will, therefore, grant more strength and speed than the others would. It's also not as steady of a power boost as you say it is, where, even in the 8th gate, they only gain just another 10% of their chakra. There are functions that go along with the release of the gates besides extra chakra.
1st:10% of chakra + they gain the use of all of their muscle power
2nd:10% of chakra + increases stamina
3rd:10% of chakra + increases blood flow and allows information to travel at a much faster rate along the nervous system(seems similar to raikages lightning armor, but probably less powerful)
4th:10% of chakra + increase in strength and speed
5th:10% of chakra + increase in strength and speed
6th:10% of chakra + increase in strength and speed
7th:10% of chakra + increase in strength and speed
8th:10% of chakra + causes the heart to beat at maximum power and exceeds the power of all the other gates
In the final gate Not only does he gain the rest of his chakra, but the heart also beats at maximum capacity. It is not speculation and is written down by Kishi as well.
About the quote, he will surpass all of the kage in speed and strength. It's not just a general statment about overall power as the gates grant speed and stength to the user. When saying greater than the kages, it means that he will surpass all of the kages in speed and strength, including speed demons(A) and monster strength freaks(Tsunade). The logic in this sentence goes like this:
When the user opens all of the gates, the user will be temorarily granted power greater than the kages. (Since gates grant both speed and strength, this is what categories the user will surpass them in.)
Since A is a kage, he will be surpassed.
In what category?
Since the gates grant speed greater than the kage, he will be surpassed in speed.
Therefore, Since A is a kage, he will be surpassed in speed.
Since Tsunade is a kage, she will be surpased
In what category?
Since the gates grant power greater than the kage, she will be surpassed in strength.
Therefore, Since Tsunade is a kage, she will be surpassed in strength.
I've been saving this explanation here for a long time just to show you good sir.