Just my 2 cents.
Laurie is often "out of position" in defence at fullback. His attack has generally been ok. He is quite willing to jump for a high kick. I don't think bravery or effort are an issue. Positioning, however, is. And if he is in charge of organising the defensive line, this could lead to even bigger problems. What he IS, is quick & agile. Now, if he is playing a support role in the halves, running off either Hastings through the middle, Douiehi out wider or any of the forwards that can offload (Papali'i for example), Laurie could be quite dangerous.
Doueihi is a big body. I've always thought we was a bit like Scott Hill, the former Storm 5/8. He's not as fast as Brooks. BUT, he is more decisive, and being a bigger body, he can offload when he runs (see Laurie above). His kicking game isn't as rounded as Brooks, but in the same breath, Brooks hasn't been a consistant kicker for the club for a couple of years. Not sure we are LOSING there. If Doueihi is getting the ball more as this suedo-halfback role, I expect a more direct attack as I'm pretty sure he'll run often.
Hastings at lock offers the creativity and control we probably lack through Doueihi & Laurie. His kicking game is only ok. His speed, as has been discussed ad nausium, is only ok. But he does scheme well. We will LACK an effective hit-up runner at #13 if Hastings is our guy, and more importance will then fall on having props that look for 2 runs per set (Hello Klemmer) to carry the weight.
Staines at fullback works IF his positional play is an improvement on Laurie. Is it? Don't know. But the reports are promising. He's pretty quick, so he offers that as well coming back. His defence is no better than Laurie. So long as he isn't missing when grubbers go into the in-goal, in my opinion, this is potentially an upgrade positionally (although I believe Laurie is a superior footballer).