When my car went to the stealers and I drove it back, it felt sluggish, horrible to drive and seemed to need to use more fuel. Turns out they had set the tyre pressures to 30psi all round. It was crap. Returned it to the recommended pressures of 37psi and it became light and agile again.
As for the tyres, you don't have to bin them but keep them to one side and use them if you have a puncture, or as a stop gap between tyre changes.