Update: I found out N5 is the Firmest core they produce, with the exception of rare production errors which can be high as N7.
My second question, is do they make solid N5 core 8"? Because most places only carry 6" cores. Since, I plan to use a 6" core alone, would it bottom out on me? And if 8" mattress is required, would a solid 8" core work better than a 6" and a 2" topper?
Yes … there was some blended Talalay that was available recently that was in the 50 ILD range that were being sold by Mattresses.net but they are no longer available. Dunlop will tend to be firmer than Talalay in a similar ILD range.
No … all the molds are about 6" and there aren’t any 8" cores. It’s very unlikely that a 6" core would bottom out unless you were very heavy but it would be firmer than a thicker mattress. Some people may “feel like” it is bottoming out because it would feel firmer than they were used to even though it wouldn’t be at its maximum compression. There is more about the effect of mattress thickness in post #14 here.
It’s really a moot point since there are no 6" cores but this would depend on what you mean by “better”. For some people an 8" mattress would be a better match for their criteria in terms of PPP and for others a 6" mattress would be the better choice. If you had a 6" layer and an additional 2" layer on top of it that was the same ILD it would be softer than a single 6" layer that had the same ILD because thickness ILD are two of several factors that can affect how firm or soft a mattress feels (see post #4 here). If everything else was equal (same type and blend of latex in the same ILD’s) … a 6" and a 2" inch layer that is glued may feel a little bit firmer than two loose layers (see post #15 here).