Basically said I just want to have a fast computer where startup of OS X and programs is a matter of seconds. External My Book USB 3.0 8TB I do have already for my backups and most files.
I also would like to install Windows 10 to play GTA 5 Online on the iMac.
You won't be able to run GTA 5 in 5k with decent performance, but should run well in 1440p. If you want to try it in 4k, the M395X would be your best bet, but might be too slow regardless. For lower resolutions (and therefore texture sizes), the M395 or M390 aren't taht much slower.
Windows does benefit from a pure SSD compared to a Fusion Drive, since it will only get HDD speeds otherwise (unless you use an external SSD for Windows). But it runs fine from a HDD as well, it'll just take a little longer to boot. Loading times in GTA don't seem to depend too much upon IO-speed. An SSD will be faster, but not by much. Once the game loaded, there should be very few situations where you'll notice any difference.
But if you plan to switch frequently between OS X and Win 10, the SSD could be worth it because of the faster boot time.
If you are planning to use an external drive anyway, 1 TB SSD is quite excessive in most cases. While the internal SSD is extremely fast, overall file operations always depend on all the involved storage modules. Copying photos from an SD card to a 1.5 GB/s SSD is just as fast as copying to a 200 MB/s HDD.
As for the CPU, it all comes down to what you are planning to do with it. You mentioned starting programs but it depends on what programs these are. For most programs (browser, typical office tasks, almost all games, and so on), you won't get much extra performance from the i7, and starting programs rarely use excessive multi threading.
It's a different story if you frequently do computation heavy stuff, like rendering and encoding videos, then you can expect a performance advantage of around 50%.
But since you asked whether the top specced iMac will be fast enough rather than whether it's overkill: generally, it's quite a fast computer. Ther are some specialized tasks where it won't be the ideal solution and especially the GPU is more on a medium than a top notch level compared to what's on the market. But unless you are expecting stellar gaming performance, I don't think a normal user can be disappointed with the performance of such an iMac.