I currently use Garritan CFX Grand on an Intel Mac using the standalone app and it runs perfectly . It is by far my favourite VST. I plan to upgrade to a Mac M1 later on this year and am doing some proactive testing prior to upgrading.
I understand your will have to use Aria Player if you want to run Garritan natively on Mac M1. (I discard the option of running standalone player using rosetta , as it is workaround rather than a proper solution; I had too many issues with Cocoa in the past to go through the same painful experience )
In the testing so far, I have noticed the following:
(loading time) : Similar . I use a Samsung T7 fast SSD drive for the libraries.
(Memory requirement): similar
(Latency) : similar
(Tuning) : all parameters that exist in standalone are avail in the Aria Player, but they have been rearranged differently ( Actually, in more logical way). there is even more parameters to monitor performance in the settings view.
(Reverb) : I can't get any reverb to work . Convolution Reverb doesn't seem to have any effect and Ambience ( which I assume is the also reverb) doesn't seem to work either.
So after testing, all works fine except reverb. The fact is I don't use any additional reverb with Garritan CFX, as the ambient mics capture the natural and original abbey road studio reverb so perfectly that it is nearly criminal to change it as I don't think any VST has recorded it so well, but it could be a problem for arrangers who want specific reverb types for cinematic effects
Bottom line, i really hope they will fix it , I have open a ticket to report it .