If you are ultra profitable then the difference between paying market rate and top of market rate might be double the salary and that would still be a rounding error on a spreadsheet somewhere.
I guess it also depends how sensitive your output is to the quality of the developers, lots of enterprises I've worked for you'd be hard pressed to tell the difference between a competent developer and an excellent developer since the end result is from a distance pretty much the same in that case it would make no sense to pay more than will get you a bunch of competent developers.
My impression from the outside is that janestreet does see that value.
Some what correlated I love it when companies say "We hire the absolute best developers" and you ask what they pay and market-rate is the answer.
JaneStreet remains fascinating to me though because while they do programming and I'm a programmer we are almost different species because of the problems we solve - which makes it interesting to me.