That is not how PCI-E or SATA works. Your drives will connect at whatever SATA speed standards the card supports. for connecting to drives (SATA 1.0 = 1.5 Gbps, SATA 2 = 3Gbps, or SATA 3 = 6Gbps). Getting data off or onto the card is an independent operation by PCI-E bus using the maximum combined bandwidth of all available real slot lines. But shortage of PCI-E bandwidth (lanes) can make communication with drives pause when onboard buffers fill up.
Quite often only certain combinations of PCI-E slots can operate at the same time. Extra slots are there merely to present the same lanes in a different mechancial intereface option. Furthermore individual PCI-E slots quite often do not include all lanes the connector supports, but instead provide more mechanical support or allow high end cards to operate at lower bus speeds. So x8 slots might have only x4 real lanes. PCI-E version changes the speed of individual PCi-E lines in a way similar to SATA version increments.
I will note that x4 PCI-E 1.0 slot actually provides 8Gbps bandwidth such that when talking to 1-2 SATA 2.0 drives that there is sufficient bandwidth for uninterrupted max performance. That is assuming non-RAID, RAID 0 or 1 operations and that the CPU can manage all striping "calculations" if software RAID 1 is used. So really only higher RAID options talking to many dirves at once are slowed. Even then disk operations may not be slowed at the card and bus. If your advanced RAID is software driven, the load of RAID on the CPU quite often throttles disk I/O back so far that the card itself can keep up with what has become a CPU limited I/O process.