Yes, in PCI-E 1.x versions each lane have 250 MB/s speed for each direction, 500 combined.
For SATA channels 300 MB/s - such speeds per channel on regular basis are reachable only with premium-class SSDs. For traditional HDDs even an enterprise 15K rpm models is practically capable for about 100-120 MB/s sustained rate at best - so dual-channel controller without dedicated memory buffer is not bus speed limited.
