Hi eges,
Your question is much too general and dependent upon your own person interpretation of what “feel” is to you and how it would relate to another item to which you had also tested. As I mentioned in my previous reply, there are entirely too many personal variables, unknowns, personal preferences and differing sensitivities for me to be able to predict how one item might feel differently to you versus another.
If you’re comparing specifications to the Beautiful and halving the bottom layer, you’re eliminating 50% of the base foam and 25% overall of the foam within the mattress, so most people would certainly notice the difference, but how much they would notice would be very individualized. Overall the feel would tend to be a bit more plush, and someone with a higher mass may notice this difference in support/alignment more than someone who was of a lighter mass. All layers within a mattress work together, not sequentially, so it would be an overall perception of comfort. There are many component mattress systems that use three 3" layers of latex and people are very happy with these systems, but how it might feel to you would of course only be known thorough your own person testing.
Making more “educated guesses” about layering combinations or changes that may work for someone is really a process of “differential diagnosis” that relies on using probabilities and some trial and error in combination with one’s feedback and assessing how their actual symptoms change and either get “better or worse” with each layer combination that has been sampled. There is more information in post #9 here about the different ways that one mattress can “match” or “approximate” another one.
In the DIY link I provide earlier, post #15 here and post #5 here and post #7 here and post #25 here would also be worth reading about making layer changes and how they can work together to change feels.
You may have found an older post where 21 was the listed ILD, but it has been confirmed for a few years now that this layer is indeed 15 ILD.
Beside the information I linked to earlier in this reply, you’re also changing the type of Talalay (natural versus blended) which will slightly change the feel and characteristics of the mattress as compared to the Beautiful, but these changes would be smaller and some people don’t notice them as much as others. Having the extra 3" support core isn’t “necessary”, unless you were trying to get as close as possible to the specs of the Beautiful, in which case it would put you closer to that item. But “necessary” would come down to your own personal testing and results, as I described earlier. When designing your own mattress, I recommend to use the specifications of something you have tried and liked in person as a good starting point, as you already have experience with that and engineers have usually found a combination that works well as a combination, The further you get away from a know configuration, the more you increase uncertainty, which you may or may not enjoy as part of the DIY process. 
Phoenix