a) look at current recipes for what it is that gives them their character
That's pretty much where I would start from. Take Rogan Josh as a random example, I'd take a look at a very wide variety of traditional recipes for this dish and look closely at the common component parts of it, but I'd also look at the uncommon parts too as well as making a few of the dishes and tasting them and go from there. Unfortunately it's going to a long slow process!
...today there are a lot of duffers yet some have really improved and produce this "best" curry. alas they are few and far between. the price that they charge is not a measure of the success in fact more than often than not the opposite.
Indeed.
But that simply proves that
some current BIR's have maintained their standards, they may be long running restaurants that have not adopted new ways of doing things, cut corners etc or they may simply have long standing experienced owner/chefs who control all the cooking.
Standards and quality in restaurants vary considerably across all sectors it's not just restricted to BIR's. You get good, bad and average restaurants serving up all variety of country cuisines.
Just because the vast majority of BIR's have adopted streamlined ways of doing things doesn't automatically mean they all have.
Let's face it, you can a good and bad Indian dish from the same restaurant depending on what time of the day or day of the week you've gone in to get it!