If we did add more RAM, would it be the effective_cache_size setting that we would alter? Is there a way to force PG to load a particular table into RAM? If so, is it actually a good idea?
Have you had a look at EXPLAIN (ANALYZE, BUFFERS) for the query?
Pay special attention to "Buffers: shared read=NNN" and "Buffers: shared hit=NNN", if you're not reading any buffers between runs then the pages are in the PostgreSQL shared buffers. By the looks of your config you have 10GB of these. On the other hand if you're getting buffer reads, then they're either coming from disk, or from the OS cache. PostgreSQL won't really know the difference.
If you're not getting any buffer reads and it's still slow, then the problem is not I/O
Just for fun... What happens if you stick the 50 UUIDs in some table, analyze it, then perform a join between the 2 tables, using IN() or EXISTS()... Is that any faster?
Also how well does it perform with: set enable_bitmapscan = off; ?