A few thoughts of from a Newbie poster to the site but cooking too long to remember!, probably long ago covered in much depth by seasoned members and those much more experienced than myself, but I will throw them both out anyway & see what happens or if its of any help.
Like myself, we are all searching for the Holy Grail of BIR.
That elusive aroma, a flavour which seems to be alluding almost everyone as far as I can see from my recent forages through the Forum, despite probably having perfected ( more so than many BIRs ) the Base in terms of ingredients, quantities and method used etc ( with the exception of whatever impact the use of their Used / flavoured oil has on the Base.)
This I suspect is more crucial to that 'taste' than any other singular ingredient.
That oil I believe is why all of us are unable to push our curries beyond the point most of us have reached.
We all know oil is the most perfect carrier of both aroma and flavour.
Hence its use since time began as a carrier of Scent / Perfume and as a carrier of both flavour and plant / flower extracts in Essential oils used by Therapists.
Hence its potency and impact upon both a Base and the subsequent production of the final curry too.
Ive also read on a number of posts from Members that they commented upon if they allowed their spice mix or Tomato paste to 'stay frying in a little heap in the middle of the pan for 3 or 4 minutes or more, as they have observed in BIRs, then they would invariably have a burnt offering on their hands in half such time'.
Again, I believe this is down to the use of well precooked/used/topped up Oil, call it what you like, because the Flashpoint of such oil has significantly changed chemically allowing for a much different approach to frying the spices for much longer. Impossible to do with fresh oil. In addition, by the BIRS constantly topping up their 'Oil' with both new and used oil, this stops it going rancid.
This is a principle, used exactly the same way with the production of dough to make speciality breads. Here a dough 'Starter' is used to kick start the fresh dough. The oldest Starter known is almost 100 years old & belongs to a Sourdough Bread Bakery in Boston USA.
The Starter began life 100 years ago as a left over piece of dough from the first bread batch the Bakery made. A bit of it was added to the next batch and a little from the next batch's dough added back to the Starter and so on and so on down the years.
You can guess the flavour. Impossible to replicate it with new dough.
I think the same principal is at work here too with both the Bases and final curries.
Finally, even though they are not anywhere in the same league, the most ordinary Chinese Takeaway NEVER uses fresh oil unless they pre flavour it by cooking at least Ginger and Garlic in it. Even then I know from personal experience they muck about by using reclaimed oils aswell.
Finally, and Ive obviously gone off on a big ramble tonight, I think the pan used to cook the final curry has a fair impact too on final taste.
Ive read time over about that illusive smokey flavour / aroma. I think maybe 85% of that is down to the oil as described above, the other 15% the Pan.
Ive used Smoked Paprika to various degrees of effectiveness in trying to get that element into the curry. However I believe a VERY well internally seasoned, patinated and heavily blackened and used pan ( never washed or cleaned, just brushed down and let the next meal's flames sterilise it ) goes much towards the final 'smokey' taste. A clean shiny pan will just never hack it.
Thats it, fingers are sore. Hope it was of interest.