There was an firmware update (1.7) for the mid-2009 MBP which fixed a problem where it couldn't do 3Gbps (SATA2). But this wasn't "silent" as information on it has been available on the Apple website since 2009.
https://support.apple.com/kb/DL853?locale=en_US
This firmware update came on June 22, 2009. The mid-2009 MBP was introduced on June 8, 2009. It doesn't hurt to ask people who are only getting SATA1 speeds on the mid-2009 MBP to see if they have this update (this is the only update for the mid-2009 MBP, according to Apple: https://support.apple.com/en-us/HT201518 ). But I would guess that most people have this update.
Now, compared to what's happened recently with regards to documenting EFI updates, there's quite a bit of information on the earlier EFI updates.
https://support.apple.com/downloads/macbook_pro_efi_firmware_update
Other than the 1.7 update for the 2009 MBP, I don't see any which address the SATA speed issue (I checked up to 2.5). So if you have information about some other firmware update which either fixed the SATA speed issue or updated the AHCI, please post links.
In terms of the AHCI version, I couldn't find good technical documentation on the MCP79 from Nvidia in a quick web search. It might be lumped with the GPU since it was part of the chipset. If the AHCI version was changed, it was either in the Nvidia firmware or the MCP79 supported both 1.2 and 1.3 from the start. I don't think that's the case. It appears that the MCP79 was 1.2 and the MCP89 was 1.3. In my late-2009 Mini (MCP79), for which Apple did not issue any firmware updates, it's AHCI 1.2. Now I suppose Apple could have ignored the late-2009 Mini with regards to this. But the scenarios where the AHCI version was updated within the same SATA chip don't make sense to me. I could be persuaded otherwise if you have a post from somebody who has a MCP79 with AHCI 1.3 or a MCP89 with AHCI 1.2. There's a lot of data out there as when people post either a screenshot or copy-and-paste from the System Information screen with the negotiated link speed, they usually include the SATA chip and the AHCI version. Not that this makes much difference as there are people who have MCP79/AHCI 1.2 who can get SATA2 and people who have MCP89/AHCI 1.3 who can't (with specific SSD's). On my mid-2009 Mini with the MCP79, the HDD shows a negotiated link speed of 3gbs (of course it can't transfer data that quickly).
Fair enough, I remember that update off the top of my head, but none of my devices at the time were covered by that update, as far as I can recall. I no longer own the MCP-equipped Macs.
I can't tell you definitively if the AHCI update fixed the issues or if the SSD firmware updates by each manufacturer on each model worked the magic. What I can say is that the problem is definitely not resolved, as there are still recommendations by the manufacturers of SSDs against using a particular SSD on these Macs. The one I linked to in the above post regarding OCZ/Toshiba's statement outright saying that the TR150 series is not compatible with these Macs. That was posted by Toshiba/OCZ on June 7th of 2016, which is not long ago and certainly well after the EFI firmware updates to the Macbook Pros. Various other forums and websites leads me to also believe that this issue is far from resolved, given these machines are quite old.
So back to the topic, I just urge caution.