I have exactly the same problem on my starlet with a T25 on , 1mm feed hole , 19mm outlet good drop to factory sump fitting . Unfortunatlly I can't tell you how to fix it as I haven't managed to fix mine yet either
I too am almost certain that its a draining problem , but I have checked the sump for presure buildup (none) .
My plans are to
(1): fit a presure regulator on the feed line and see if this helps,
(2): trial a pump on the turbo drain to see if this improves it
(3):try something else if these ideas don't fix it
I also have had 4 diferent turbos (2 freshlly rebuilt ) on the car and the problem stays the same annoying bloody smoking .
good luck sorting your problem