This request has been added to the tracker:
https://winscp.net/tracker/1680

I'm sending you an email with a development version of WinSCP to the address you have used to register on this forum.

OK, we'll consider this. If we implement this, I'll contact you.

I want synchronize two folders with too much sub-folders/files, so option "preview changes" make no sense and would not work (see memory error report).

That task needed this time ~15 hours and end silently. I would like to see a messagebox by finished job with a summary how many new files moved or deleted or overwritten and how much time the complete task needed. ;)