Is intel's integrated Iris plus 645 1536 MB much better than the Iris 6100 1536 MB

In your other thread you mentioned gaming and total war titles. Iris graphics are still integrated graphics. They're not what you want for gaming.

Even if they're 100% better than your 6100 graphics, that's like saying this car with 2 wheels has 100% more wheels than this car with one wheel. i.e. it's still rather lacking...

Check out:

Or this:

You can see Iris graphics are, while decent for integrated graphics, still many, many times weaker than even the most modest modern entry level graphics card for gaming, the GTX 1650.