Answering your questions:
There will be very little impact in terms of performance. These SSDs come with a number of features that enhance the reliability and performance all together. Please understand that without AHCI you will not be able to get any smart detail readings such as the ones found on the Intel® SSD Datacenter Tool.
As for the garbage collection, it will help but does not replace TRIM.
Garbage collection collects used pages into entire used blocks, thus making more space available for immediate writing and reducing writing amplification. When a file is deleted, the pages of the file are not touched. It is done with the directory entry in the filesystem.
All Intel® SSDs have TRIM and Garbage Collection as standard features. The Intel® SSD DC S3500 and other data center series have additional space so the data can be moved and the impact of not having TRIM is down to a minimum.
Let us know if this helped to answer your questions.
We will then give it a try.