AHCI stands for Advanced Host Controller Interface. It makes Native Command Queuing (NCQ) along with hot-plugging or hot swapping through SATA Serial-ATA host controllers possible. NCQ is one of the important features of AHCI for SSDs. SSDs can process requests faster than HDDs. It can process so fast that the SSD could end up waiting for work. NCQ allows the OS/controller to request up to 32 simultaneous requests at once. So you basically get more performance from your drive over older IDE mode.
Here is a 256GB Vertex 4, you should be similar in reads to it, but a little lower overall score.