You are wrong simply because all you are saying is against how an X86 architecture works; nothing less if you trust a benchmark and the words of a brand offering “a commercial product with another commercial product”,probably you will never reach a good level of performance.
If you think that is enough to have 2 video card to have the double of the performance you probably never study how a system actually works.
In every system where the performance are a critical point a kernel UNIX or Linux is in use, that’s what i can tell, and since you are talking about benchmark you are considering, again, fancy stuff from the marketing language and from what the brand want you to buy.
Never the less, GCC is a generic compiler suite, and it’s true that a good compiler can affect the performance but not as a really good driver, GCC is a good and old suite that can be surely improved but i personally do not expect an huge gain in terms of performance from the version A to the version B, a compiler it’s not what offers you the cutting edge performance that you need and also plays a minor role in how an X86 architecture works.
Just to say something, with a bad driver you can turn a 16 core cutting-edge machine into a 486 or worst.
If you want to stick with Windows is fine, but the industrial standard for the ones who are looking for performance is Linux or BSD, that’s all, and before judging an answer please open a book about an X86 architecture to see how you computer, your hardware, your kernel, your drivers, works.