There have been many debates on whether HALO will increase safety or not. From what I could found it will increase safety by 17%.
From FIA presentation on HALO showed that in every incident in which a driver had suffered a head injury, or had narrowly avoided one, in the last 20 years, the halo was either a positive safety improvement or had a neutral effect. Also allaying the drivers concerns on visibility.
Lewis Hamilton view after this presentation:BBC
"I take safety very, very seriously. The interesting thing is that while the halo system does not look great or in the racing spirit, the chances are 17% better of saving the driver's life.
"But it can still be improved so at some stage we will have canopies and then it will be 100%."
Lewis who was criticizing HALO of it's look changed his view after this presentation.
From statement of the FIA:formula1.com
Having developed and evaluated a large number of devices over the past five years, it had become clear that the Halo presents the best overall safety performance.
At last "Will the HALO protection system improve safety in F1?" yes it will definitely increase safety to some extent.
References: Bleacher Report, halsteadgazette, f1powernews, willthef1journo, MotorSport, Sports Illustrated