They are not RAID cards, that would require an actual RAID card
These act pretty much like a networking switch but for SATA/SAS drives. so yes the maximum bandwidth of the original port is shares between all the new ports. This shouldn't be a problem though, they intended for situations where speed comes second to number of drives and volume of storage.
You might have to watch out for compatability issues though, some controllers and RAID cards don't like them. Others fully support multiplier cards however.