Boost control via WGDC in the EJ205 ROM leads to good boost in one gear and messed up boost in every other. Or good boost control at one ambient temperature and different when the weather changes.
WGDC that's good in 4th will result in abysmally low boost in 2nd and 3rd. WGDC that's good in 3rd will result in boost spikes in 4th and 5th.
People are recommending tuning TD, because that's how you do it right. Once you get target boost and TD right, then WGDC max and initial is used to keep from having an overboost spike when you slam the throttle at 5000 RPM... something well over the boost threshold RPM. Tuning the whole boost curve by WGDC will result in slow spool, or significantly varying boost in different gears.
You can half-ass it in the ROMs that have per-gear compensations and be okay, but the EJ205 ROM doesn't have the WGDC control to do it.