Q: does FMaj15 sound dissonant? (it contains E F F# G)
A: Fmaj15 is not accepted as a legitimate name for a chord anywhere i have ever been. if it were, you would need to call it #15 in order for it to include F#. and yes, a chord which includes e, f, f#, and g would be dissonant as hell and would really only belong in a horror situation