quite right. Hoever, almost no HDD's actually manage to move data to/from the mobo faster than a 150 Mb/s rate anyway, because the drive unit itself can't provide data to the buffer RAM any faster. Much of the time the communication system in a SATA II unit is waiting for data to move. So the impact will be small.
However, RAID1 is best done with well-matched drive pairs. If the drives are that different, do their other characteristics match well? Capacity, rotational speed, head seek times, average latency, buffer sizes - all those things that need some match. As chunkymonster suggests, any mismatch will be resolved to accommodate the slowest performer.