The performance isn't similar because they each have a different BIOS installed that tells the card how to behave. Each GPU is tested prior to going to the card manufacturer so they can build a PCB and shroud that accommodates its tested performance. This is how Kingpin ends up with cards that are able to hit 2580 MHz. Though they all do have the same chip, each has the capability of being just a bit faster than the next all the way up to the ASUS ROG STRIX RTX 3070 OC. All this information is readily available at TechPowerUp. It is an amazing resource tool for GPUs, CPUs, and reviews. It lists all iterations of the 3070 as well as almost every other card that is currently available.
That being said, right now there are 4 versions of the 3070 ROG STRIX, but technically only 2 since each comes in a different color (one black one white). As far as I can tell, there is no regular ROG STRIX, only ROG STRIX GAMING. This leaves the ASUS ROG STRIX RTX 3070 GAMING and the ASUS ROG STRIX RTX 3070 GAMING OC. with OC being the only difference (for "overclock"). Though they both use the same GPU, the OC has a significantly higher built-in boost clock (1905 MHz vs 1725 MHz). They are otherwise all but identical. It just means the OC versions are the pick of the litter so NVIDIA slaps a different BIOS on them that allows for higher speeds without the user having to manually overclock the card.
The wonderful thing about these particular cards that sets them apart from the competition or other ASUS GPUs is the heat sink and fan shroud. It is superb for keeping the card nice and cool. I'm running the ROG STRIX RTX 3070 GAMING OC myself and I've never seen it go over 67°C even running FurMark for over an hour (I run it at +165 core clock and +1200 memory clock with MSI Afterburner). Of course, the cards are designed to automatically lower the clock speeds as the core temp increases, so the moment it goes over 65°C the clock starts dropping in bins of 15 MHz.
So yes, use TechPowerUp whenever you are searching for a card to buy. All the information is right there.