Right now the max value for "EPGP:CanIncGPBy" is "99999", however the GP value of TBC items can reach higher values, such as "Sunfury Bow of the Phoenix" having a GP value of "119428" for hunters. Manually changing that number seems to solve the issue, but I am not sure why this number was chosen? Does changing this have any side-effects? Perhaps there should be an update increasing it if not. Thanks

I think you should modify the base item level and re-scale GP.

I don't know why it was "99999". I think it is fine to modify it manually. Large EP/GP may not displayed correctly on the UI.