That's a lot of effort to fit it in. It would be interesting to listen and see whether WM8741 makes a big difference over the WM8740. Personally I wish Fiio chose a more modern D/A chip for the X3, since WM874x series is getting a bit dated. My CD player from 2007 had it. Technology at the time couldn't handle 192kHz at 256fs mode, while any newer DACs can.
For the opamp, what's the reason for using BP (DIP) rather than using two SOIC chips like 627AU on a small adaptor board?
Subjective perception aside, OPA627 is a not a good choice for output stage opamp - not particularly good voltage swing, limited capacitance load capability, plus a 55ohm open loop output impedance. The better way of using OPA627 is to have a buffer stage after the opamp so it will never need to drive any load directly.
Me too would use OPA627 for an I/V or something, but not for driving headphones on a 5V supply. It was not made for that kind of application. It will certainly make it sound unique and different though. Very warm and thick. I think the AD8397 that's originally used in the X3 is a good choice to suit a wide range of headphone demands.