The window size and scale factor is set by the source host, you can't change that. That's unlikely to be relevant.
Port forwards don't rewrite source ports. There isn't a difference network-wise between allowing traffic in from the Internet on 1:1 and rdr (port forward). Either way you're strictly rewriting the destination IP (though with rdr you can rewrite the destination port that's only if it's diff outside vs. inside, eg from the Internet you have a web server listening on port 8000 but it's on 80 internally, that doesn't sound like it's the case here, and wouldn't be relevant either way). That's different for outbound traffic but that's not what you're looking at here.