I have used AHCi very long time to hot-plug external sata drives into a back-plate that came with a Gigabyte motherboard I once had.
Never occurred to me most have never enabled it.
It is very handy you can get rid of the remove device thingie as all my bare drives are for archive only there is no problem with write-caching plus it gets rid of all the sata controllers in devices.
You can also disable hot-plug on any sata controllers you like. say the SSD, your primary Sata backup and your DVD leaving you a few to hot-plug.
So AHCi should not be thought of as something you do with just SSD.
As I said KISS, With SSD and Sata for that matter keep a small OS partition and do a 3 monthly zero write for SSD ( for heavy use ) 6 months ( light use )
To restore that near perfect image then takes barely 10 minutes all up for me.
When your "Well worn re-installed with many apps I did not need SSD3"
does not run 7.9 WEI anymore when you do fresh install W7... do a zero write with PMagic ....and that is why you want that near perfect image so you back up and running in minutes , not hours.
Then if you go on and play with many apps as mrpowcz you can knock yourself out and wreck the partition ( I have on more then one occassion ! ) .
With Trueimage you can go back to your "near" perfect sim image in 3-6 minutes.
Full 120GB SSD backup would take 20 minutes and 30GB+ space.
Why I run a 30GB OS only they take 2-3 minutes to backup and are 7GB in size that will fit on a 8GB flash.
So for me the dilemma of OP I would at least try this.
1. Clean house on your Sata.
2. Do a image in case step 3 fails
3. You can edit registry to allow install of AHCI drivers 50-50 in my experience.
4. If it works okay after test and checking everything I would do a new image for the SSD.
5. If it failed to install I would just restore the first image.
Back to square one but it is worth a shot.