My claim is, given ×2 mining speed everytime the next wave is called early, there will be no difference in the resources earned, except the time it needs to take them.

Consider these scenarios:

For simplicity, lets assume a miner gives 1 resource every second.
Suppose in a wave, it takes 15 seconds to spawn all enemies and a 10 second wave interval.

First scenario: The next wave is not called early.
In the first 15 seconds where enemies are spawned, you earn 15 resources, then in 10 second wave interval, you earn 10 resources, then in the next 5 seconds of the next wave, you earn 5 resources, a total of 30 resources in 30 seconds.

Second scenario: The next wave is called 5 seconds early.
In the first 15 seconds where enemy is spawned, you earn 15 resources, then in the first five second before you call the wave, you earn another 5 resources, then the next wave is called. Since there is 5 second extra time, there is a x2 mining speed in the first 5 seconds in the next wave, so you earn 10 resources in the first five second of the next wave, a total of 30 resources in 25 seconds.

You still earn 30 resources, but at different timespan. But this doesn't mean you earn more resources, since you earn 30 resources at the same point of the game. In general, once the x2 mining speed stop, you still earn the same resources as if you didn't call the next wave early.

That's why I think the game should give a little bit more mining speed every time the next wave is called early. Like, instead of ×2, make it ×2.01.

Or add "mining speed boost for early wave call" in the research tree, which add 0.01 mining speed boost for early wave call for every level of research. So at level 0 the mining speed boost for early wave call is ×2, at level 1 the mining boost for early wave call is ×2.01, at level 2 the mining boost for early wave call is ×2.02, and so on and so forth.

I'm sorry for the long explanation. I hope you consider my idea.