Focus+restraint I think is testable, considering that when you run berserker's rage (25% damage when at max fury) the damage WILL apply to boulder toss even though it depletes your fury before it lands. I tested that out a while ago and what happens is the berserker's rage damage bonus doesn't fall off until the boulder lands, making it keep the bonus.
Yeah, thought about that combo. But then I reread the set bonus, it says it only increases the damage output while being on full resource after you have spent resource. So, since you deplete all your Fury with one boulder toss, I don't think it will count sadly, since it doesn't hit immediately.
Also taking into account, that Unity is a must have, if you desire high Greater Rift ranks + Ring of Royal Grandeur, if you want to be able to have 3 set IK + Reakor 5 set bonus.
But I definitely am going to test them out at some point
Back when I completed raekor I played it with boulder toss using 300th spear/sunkeeper it could deal with crowds extremely effectively as the boulder spam keeps your hp high and bloodshed ruins the crowds.
Now with the 2h buff... I can see a boulder breaker or other 2h weapon to buff up your single target being the most useful, as the damage will balance out with the 50% ancient spear buff as well as the crit simply due to the weapon damage.
If you want to run without rorg you will need the boulder breaker and no other legendaries like vile wards to buff up the sets, so I think that will be a must have. Unities for the toughness... There probably isn't another usable set of rings for this build.