Ah, I didn't know the source was available for the assembly.

I've attached how I implemented it. Added an int property called TransferSpeed, setting it as zero when initializing the TransferOptions, then later if TransferSpeed is greater than zero it will add the switch.

This is not supported with .NET assembly yet. But you can easily add it as the assembly is open source.
See similar example:
https://winscp.net/forum/viewtopic.php?t=12452

Anyway, this request has been added to the tracker:
https://winscp.net/tracker/1019

Is it possible to set the transfer speed limit for synchronizing directories using the .net assembly?

In the output it looks like it's running the synchronize scripting command, which has the -speed switch. Is it possible to add the switch to the command somehow, or is it not implemented?

Cheers