I am not certain, but I would speculate that any serial port built into a laptop would be a 'native serial port' since it would be built into the motherboard. It would actually be more expensive for the manufacturer to put a USB-serial adapter. If it was a USB-serial adapter, then you wouldn't be able to access it in the same way from the BIOS.
Is there much of a speed increase with a native serial port to a USB-serial adapter? It seems that the main bottleneck would be whatever baud rate the RC wants to talk to you at, and I have never heard of a USB-serial adapter that didn't support the standard array of baud rates.
[edit] To Adam's point, serial transfer rates are measured in baud. One baud is one bit per second. There are 8 bits in a byte. 20000 is the
maximum the standard officially supports.[/edit]
[double edit] I just noticed that you were talking about a Dell port replicator, not a Dell laptop. I have no idea, but I still don't think you're going to see any amazing speed increases[/double edit]