I think that it is still worth scavenging. You will have a higher flow rate with a T-piece compared to a circle and, while during early mask induction there may be escape of gases around the mask, once it is firmay applied it can be mostly scavenged. I prefer not to have several litres per minute of 8% sevo floating round my head or even 3-4%. Once on the ventilator, all gases are automatically scavenged, except for an unavoidable leak around the tube.
I still remember halothane in the days before scavenging and am a convert. Even when I go to Nepal to do cleft palate surgery, we passively scavenge.