I haven't seen an ACL in use longer than 500 lines.
Object-groups are for administration only and are expanded to their full length in memory for processing. They should not have an impact on perfomance during normal operations.
The turbo ACL feature hashes the access-list into a lookup table. The documentation I read is that a packet can be determined to pass or fail the ACL within 5 checks of the table. Therefore, long access-lists with turbo-acl have a drastically reduced effect on the performance.
Unfortunately, I do not have a references for specific performance of such long access-lists.
If you find such references, can you please post to this thread?