ACT! performance suffers heavily when the attachments folder exceeds 10K files (mainly Windows issues reading the FAT). As it's the number of dir items, not the total size, even shortcuts impact this.
Can you look at allowing subfolders - either automated to create a new sub every X-number of files or (better) folders based on a user selectable field (like ID Status or the contact when it's saved).
If some ability to manually select the folder, this would also have the security benefits of being able to secure a folder by Windows/Domain ACL and also select folders to be excluded from Universal Search - while still supporting sync/web unlike shortcuts (which are the only way now to do this) ... currently, Universal Search is a real security flaw