Although not directly an answer to your question, I do not believe that there is still a missing link to be discovered (no secret ingredient, no secret style of cooking, and so on). I personally believe that there is now sufficient information in the public domain (including on this forum) to allow the average home chef to re-create a basic BIR curry without difficulty, and to get to within (say) 95%. As to whether we yet have enough information to re-create all of the standard dishes, I am less certain, simply because I restrict my home curry preparation to a basic Madras style, and I am less certain whether I could re-create (say) a Chicken Bombay or a Chicken Bhuna with the same degree of accuracy and success. As to tandoori, I last cooked this for guests about eighteen months ago, using a domestic charcoal barbecue, and the recipes I used (which differ between Chicken Tikka and Tandoori Chicken) were the result of multiple refinements and fine-tuning over time. The results were as close as I have ever got to complete authenticity, and were (in my opinion) somewhere between 85% and 90%.
But to answer your real question, I am confident that the answer is "both". We have already see over the last few days that Ham's base with black cardamom is both attracting attention and attracting praise; but a base alone cannot make a curry, any more than can the final preparation and spicing.
My two penn'orth, for what it's worth.
** Phil.