You do lose on heat, though - 4x LRM-5 is 8 heat, 1x LRM-20 is 6 heat. If you want to fix that difference with extra heat sinks, it'll cancel out the mass advantage with SHS(and leave you 1 slot worse), and it'll leave you 1 ton better but two slots worse with DHS.
Also, because they'll tend to hit in blocks of less than 5 damage, the LRM-5s are slightly better at crit-seeking as well, though correspondingly worse at hole-punching.