We A3 owners were blessed with the government taking something simple and popular and making it unique and an oddball. It's what a committee does. Using locked torque converters never caught on with the 545, so only the 5600 or so A3's had them. The input shaft from the TC and pump are larger diameter with the locking torque converter, I don't know about the bolt pattern on the TC, as I don't have a 545 out (I have one in a bus), to check.
That said, the guts of a 545 can rebuild all but a couple seals of the 1545, so there is no reason a tranny cannot be upgraded with frictions and seals as long as you don't wait until it's really trashed. My whole piston melted---gone--- and I found new for really reasonable cost and am 100% confident in the rebuild. Do I need a 3L and 4L...no, but if I wanted to go highway a lot, it would help.
That said, the guts of a 545 can rebuild all but a couple seals of the 1545, so there is no reason a tranny cannot be upgraded with frictions and seals as long as you don't wait until it's really trashed. My whole piston melted---gone--- and I found new for really reasonable cost and am 100% confident in the rebuild. Do I need a 3L and 4L...no, but if I wanted to go highway a lot, it would help.