I know a few people who are doing 4 A Levels and an EPQ, which technically is the same workload as doing 5 AS Levels. They are all very hard working, however, and usually spend their free time working and revising. I guess the benefits are you can talk more about what you've got out of doing all 5 in your PS and also show the range of skills you have? It shows you have good time management as well and also means you can discredit your lowest 1-2 grades if you don't do as well in them, as unis only want 3 A Levels and dropping 2 for A2 wouldn't put you at a disadvantage.