One thing I do not understand, for example let's look at the benckmark of a mac pro 2008 3.1 8 core https://browser.geekbench.com/macs/254 on this score.
If instead we look at that of an I7 4790 k we have this other score https://browser.geekbench.com/processors/1536
So the second, mine has about twice the power, but how are these benckmarks made?
With HD video games or with export or import rendering proofs, I mean that maybe for video games, the Mac pro and lower but for video editing instead is more powerful, isn't it?
Is there a benckmark for PCs where the power is calculated based on editing with a specific software x video and a specific video card?
Are you saying you have an iMac 15,1 which has the i7 4790k processor, a Radeon R9 M290X GPU and Thunderbolt 2 ports? If you have this computer, then why are you still, almost a year of asking the same questions again and again wanted to know if the Mac Pro 2008 can be made as powerful as an i7 4790 iMac or PC? A Mac Pro 2008 is an 11 year computer and was quite powerful when it came out 11 years ago. And it is still powerful when compared to an 11 year old iMac, 11 year old Mac Mini or even a much older PowerMac PC G5 Quad because it can take better GPUs, has better I/O support. But as you probably know; technology progresses. Now even a modest latest phone is "more" powerful than that 11 year old Mac Pro 2008. If you want to tinker is one thing with the Mac Pro 2008, the computer itself is simply too old to keep up with a somewhat modern CPU like the i7 4790. And even that is no longer as modern as the current i5, i7 and i9 CPUs which have more cores and speed. Just because it looks like a tower does not mean it's powerful. It's the age of computer.
Only get a Mac Pro if you have a specific reason to run specific software like I did. Otherwise, stick with your i7 4790k and find ways to edit video with it. If it's an iMac, FCPX will run on it. If it's a PC, then Premiere will run on it well combined with a wide variety of more powerful GPUs. I'm not sure why you had spent almost a year asking the same questions over and over wanting an assurance that the 2008 Mac Pro will be just as fast as the Core i7. It will only be fast if the Core i7 has a cheap weak GPU and very slow USB 1 drives. If you give the Core i7 a Radeon Vega 64 and RAID 0 SATA SSD array or spinning HD, it will blow the 2008 Mac Pro.
Many people build Mac Pro editing machines because it's still a feasible budget approach if they can't afford an i7 or the latest Macs, but these people had accepted that there will be certain compromises. Life is about accepting certain compromises within the financial capacity of that individual. It sounded to me like you want to have the cake and eat it too, but unfortunately, you simply can't have a 2008 Mac Pro at the price point you're looking at and expect better performance than the i7 4790 machine and in the case of the iMac, a Radeon R9 M290X GPU and if it's up to me, I would put my money into a 2015 computer rather than onto a 2008 computer.