The crema definitely affects the initial taste of the coffee, that's where the concentrated notes of whatever bean you're using is going to be found.
I make a long black by:
- Pour 3/4 cup of hot water (right from the machine)
- Wait 2 - 3 minutes
- Pull a double shot into it, holding it right under the group to minimize splashing
- Let sit for about 30 seconds, serve
In contrast, I make an Americano by just pulling a double shot into a cup while also taking care to minimize splashing, then I serve it with a small steel or ceramic carafe of hot water (everyone likes a little more or a little less than everyone else).
In both instances, I heat the cup, and that's important because the crema will quickly break up on the walls of the cup if you don't. Most good espresso machines allow you to heat cups on the top of them.
Now, my testing is somewhat limited because I drink good quality Arabica beans pretty much exclusively. Arabica has some subtle flavors and it's easy to tell when they're more pronounced. Long blacks do hit the caramel notes more, in my experience. But, I usually just make myself an Americano because it's easier and the taste isn't all that different.
Done at home using a Breville Barista Express in manual mode. You don't really need a super expensive machine to replicate this, you just need the ability to pre-infuse, warm the cup, and be able to wrangle one under the group to minimize splashing.
Concluding, I think there's some substance to this. However, I'm not sure how much it really matters to someone kind of fumbling around to get their first cup of the day. It took close to a dozen tries to finally get a process down where the taste was noticeable and I could reliably reproduce it.
If you order either at your average coffee shop, I don't know that you'd be able to tell the difference, I never have.