I've done the W25Q128 upgrade on several similar TP-Links. It's a nice upgrade.
Modified u-boot is not really necessary. The stock u-boot will boot with a larger chip. It can only write to the first 4 MB of the chip though.
Instead of making large firmware images, it works to use a release image (built to fit in 4 MB) then install packages (from the release repository) at run-time. The release image contains code to auto-detect the flash size and use the additional space.
If you only copy the bootloader and firmware into the new chip, OpenWrt will still boot up (but no wifi without the ART). The kmod-mtd-rw package and mtd can be used to install the ART after first boot.
I would suggest first running OpenWrt on the stock router and using it to read the bootloader and ART from the original chip (and scp them to your PC) before opening up the case.