400s is atainable with H2/LOX, but I wouldn't put my money in achieving it with other fuels (that still provide high thrust as well). They don't tend to reach the 400 mark and development prospects don't suggest they "ever" will. I've researched literally 100 engines on this aspect. There doesn't tend to be a lot of Isp increase per fuel combi once initial development of the particular engine type is done anyway. Almost all of the Isp increase we still see in H2/LOX engines is due to increased nozzle size.
There is only one fuel combination that is more energetic than H2/LOX, but you can't use it for environmental reasons (something like fluorine/LOX IIRC).