That's 4 mm x 4 mm (more with passives, but not too bad), and hits a solid 95% efficiency at the normal load of an iphone, and is above 90% all peak loads. Heat dissipation will be fine, all parts cost about $1.20. Low EMI.

However, its still a dozen new components. It's a ~10% tradeoff somewhere and while that may be totally worth it, its not simple. Phones do not have square millimeters to spare.