ECU is struggling to cope with the setup, Aftermarket ecu would be the best way to go (as you say syvecs). My old hawk struggled (forged 2.5 on an sc46+ with ecutek map). Didn't like idling smoothly when coming to a stop either cos of the mass of the turbo. Check out the fueling before 4500 and the way it plummets then spikes before settling,
The T is a 450bhp turbo at 1.6 bar, so your bound to get a lower result at 1.3. I'm assuming its still running the MAF?
Ecu wasn't designed to run that high boost or that much power, my advice bite the bullet and syvecs it.
Different cars with different parts will get different results, but on low boost (as above graph)
Basic spec, (Was held back cos of the top mount), but was single AVCS heads, standard headers, full decat and decat up pipe, sti top mount, sti inlet manifold, 400L fuel pump, 800cc (i think) and SC46+ mapped on Ecutek by pat Herborn.
My current 2.5 is running less power, but on syvecs, you can see ont he graph how much smoother it is,