This is interesting. First of all, how were handicap differences handled "traditionally" (do you mean before komi?). If we don't change komi, then what is the difference between a one-stone handicap and simply letting black go first? Or was going first considered being one rank stronger traditionally?
To get your head round this you need to understand that the ranks we now use are a relatively modern construct. Traditionally (Edo times) ranks were limited to pro-level dans. They didn't even use komi. Honinbo Shuho introduced some lower grades for amateurs but his system was soon abandoned (for political reasons) and players reverted to the old dan-only/pro-only system, essentially until the democratisation after World War II. The amateurs started using their own dan scale then, and the first amateur 6-dan was Hirata Hironori in 1955 (for winning the 1st Amateur Honinbo - the prize nowadays is 8-dan).
Since then amateurs in Japan were able to use kyus - and did, but the lower ranks have been used with much more gusto in the west. Indeed, a number-only system was introduced by amateurs in Germany even before the war, and was either used or copied by other western amateur associations. We have seen western amateurs - very many with a mathematical background like those early Germans - obsessively try to apply rules and numbers to many aspects of go.
But handicaps existed well before ranking systems and so it follows they can have no real correlation. They were used to a very limited extent between pros but mostly were (and still are, in Japan) nothing more than a teaching tool. No doubt for that pedagogic reason, too, the stone placings were fixed - the idea of free handicap placement is another modern idea, inspired first by mathematical amateurs in Japan (and even giving rise to a book on them by a pro!). The use of these for rankings, and of komi, likewise has no tradition (or even theory) behind it.
The use of komi (mainly in trying to determine what an even game means - and that's varied a lot in the last 100 years) is likewise mainly an Japanese amateur idea, from 1751. Pros tried it a few times from the early 19th century, starting at 5 points and gradually reducing over the decades until it even reached 2 points. It only started to rise after World War II.
So you can see that trying to tie ranks to handicaps is like climbing up a greasy pole. Historical grades and handicaps both differed for pros and amateurs from modern ones, modern grades and handicaps differ between amateurs and pros (and by country). Komi has been messed up for 300 years. The philosophical drive behind rankings differs in the west and the Far East. People have different ideas on how to implement handicaps. Etc, etc.
Life is too short to worry about such things. Of course we all would like a way of quantifying how much stronger A is than B, but it seems sensible to accept it's always going to be a wild guesstimate - at least until we get a DeepRatingsZeroPixie algorithm.
