If it works, and it seems to be at the moment, then leave it be.
Api is a seasoned campaigner and is playing better than he was last year. Sure he has his deficiencies but he brings enthusiasm in attack and defence. Hastings is still learning his trade. Too easy to forget that at 21 years he's only a few months older than Kelly and less than a year older than Tom T. Api is 24.
If Hastings is handled well, he could turn into one of our very best, and the gradual step by step approach with increasing time in first grade, is giving him time to develop without copping the brutality at this stage of 80 minutes of heavy weight second rowers and props crushing him. He's destined for higher honours I believe, but incremental exposure in such a tough role as hooker (or 5/8 and half) wont hurt.
As a centre or full back, players are less exposed to the heavy forward play and can succeed earlier out wide. In the ruck its a brutal traffic jam. I think Barrett has it right.