Leave the start value as is; set to 0. It needs to be set so that the driver loads early in the boot sequence. [strike]Once enabled (set to 0), you can change the BIOS back to ATA mode and you will still boot.[/strike] Whether you can boot in either may be BIOS depenent. My Dell T110 had a BIOS update that now crashes Windows 7 boot wih AHCI enabled if BIOS is set to ATA mode. This was not the case in the previous version. The setting changed only allows the AHCI driver to attempt to find an AHCI device on your system. [strike]I'm really not sure why W7 did not leave this enabled by default.[/strike] This was learned recently; no boot in ATA mode. I guess that they assumed that you would install the OS on a machine that had it enabled; then it is enabeld by default. Oh well.
One thing that I noticed in ATA vs. AHCI, this with a spinning platter HD, is that ATA benchmarks are better on the same drive/system for sequential access read/write (~20MBps). Only random access showed improvement with AHCI enabled; I assume due to the command queuing, but I'm not a an expert on that. I have not had a chance to becnhmark my SSD to see if it makes a differnece. Currently, I only have two laptops with Intel SSDs to improve performance, and these do not have an AHCI native interface in BIOS.
I doubt trim only works in AHCI. I have never heard that and cannot see why it would other than Intel limiting it for performance reasons; again command queuing feature of AHCI could help that.