Does iPhone's new HEVC video format also halve file size for 720p and 1080p?
Yes, iOS 11 supports lower resolutions HEVC resolutions as well.

Please note that the bit rate reduction will be lower at lower resolutions. The setting Settings > Camera > Formats is a global setting an will affect video and photo capture at the same time.

Source: Wikipedia/TK Tan; Marta Mrak; Vittorio Baroncini; Naeem Ramzan (2014-05-18)