anyone knows the stock fuel pump LPH rate on the normally aspirated x300? would a walbro 255lph be more than the stock?

 

XJ8 has one 155lph / 3bar Denso pump and X308 XJR has two pumps.
I guess X300 has not higher capacity pump, so Walbro would be well enough for your needs. Single Walbro 255 high pressure pump supports 500+BHP with stock pressure. I have two of them in my car. Please check your return line, since you may see too high fuel pressures with Walbro if you do not increase capacity of the return line.