as far as i understand, lisdexamfetamine is purely a prodrug for dextroamphetamine, broken down into that and lysine in the body and because of its delayed onset and specifically because it's a prodrug it's abuse liability is supposedly lower than that of d-amp.
since its effects and pharmacodynamics are pretty much synonymous with d-amp, it has no appreciable effect on serotonin.
a higher dose, as with pretty much all drugs, probably will get you higher. keep in mind, i've never taken the stuff, so i'm not speaking from experience.