So there is no performance impact at all by processing the transport rules?

That is correct.  There are no performance issues in Exchange 2007 with Transport rules.

PS Note: I will be out of the country in a remote destination for next couple of weeks, and will not be able to response to any queries till then.  But pls feel free to ask, and I will respond once I get back.  Also, others within Microsoft and with knowledge in this area may also answer your queries / questions in the mean while.

Any difference when the connector restriction is a clustered server?

No difference.  Same code and same logic / algorithm still applies.

Is there any way of using transport rules to route mails from a specific sender (or sender domain) to a specific send connector or another smarthost than the one on the default send connector.

We just solved the problem at hand (route mail to different servers based on source rather than destination in Exchange 2007). We were able to create a 2. recieve connector and use the LinkedReceiveConnector property of the set-sendconnector cmdlet. This was only possible because the sending system is not clients, but an application on a specific server.

The questions still stands:
Is it in any way possible to route mails from a specific sender (or sender domain) to a specific send connector, so that you get (almost) the same functionality as when using connector restrictions in an exchange 2003 environment?