Will switching from Concrete Table Inheritance to Single Table Inheritance improve the performance of my queries?
If you are primarily concerned with the complexity of getting the 20 most recent records, you could either write a view that does a UNION ALL of the tables, or create an empty parent table with all the common columns and have all the other table inherit using the PostgreSQL-specific inheritance feature, to hide the visual complexity of the query.  Although it probably won't improve the performance of the query.

Changing the structure as you suggest would very likely make the specific query you asked about faster, but other queries that query only records of one type would probably get worse.  You have to decide which queries are more common and more important.