Yes it effects the sustain and also the timbre of the guitar acoustically. The sustain is something that is interesting and a bit paradoxical though because a strong body resonance can actually hault the strings vibration. This seems to be a characteristic of the good old ones. Maybe CW will chime in here because he has a fleet of different years with cutouts and non-cutout block guitars. But what I hear from the 59,s I have played, both 335 and 345, is characterized by a strong attack of the note, followed by relatively rapid decay.