According to Anandtech's SP3 review, the i5-4300U scored 2.81 in Cinebench R11.5 (13.3% better) and the i3-4020Y scored 1.63 in Cinebench R11.5 (34.3% worse).
Given that the Core M replaces Y class CPUs (11.5W TDP) losing by 13.3% to a U class CPU (15W TDP) isn't bad.
The Core M 5Y70 scores much higher in Ice Storm than both i5 (42,129) and i3 (32,327) Surface Pro 3s.