I began diving a long hose, necklaced backup on my HunterPac (Similar to BP/W) for open water and quickly got away from it and switched to a SP Air2 5th Gen.
I have a couple reactions to what's been said. First, (personally) I'm glad I didn't buy a SS plate. I'm here in Florida where it's hot and clothing is optional. I dive steel tanks and I would be grossly overweighted. I don't dive with any weight until I get above 3 mm's of neoprene. I do like the sound of the Kydex plate offered by DSS, but I wasn't aware it existed at the time of my BC purchase.
My HunterPac has a pull dump. I'm not the least bit concerned about it. In fact, I've never used it, but it's there. I look at as just another option in certain situations.
The long hose in OW is what I would describe as an "octopus" wrapped around you. I totally get it for cave and wreck penetration, but see little value in OW. For OW I think the best configuration would be the "streamlined open water" setup. 40" hose under your arm with a swivel and 22" second over your shoulder.
As far as the Air2. I don't see the purpose of diving it unless you have a good reason. For me it is a back mounted pony (required for spearfishing). The reg on the pony gets necklaced. Is this the best configuration? No. If I weren't fishing I would sling the pony and use the streamlined ow setup. But, I need the front of my body clear for loading my gun and fighting fish and I'm not going to dive with 2 regs necklaced. That would be rather stupid.
Despite what's been said, the Air2 hose provided from the factory is too long (26"). If you're going to go with the Air2 and If you can buy it without the hose and just the special fitting, do it. I had to go with a 22" LP hose and I used the 18" corrugated hose that was already on my BCD. It is not dragging or getting hung up on anything. It sits just fine at the bottom of my peck. When breathing from it I have full mobility to turn my head.
So, my setup is basically similar to the streamlined open water setup. But instead of a necklaced second, it's on my inflator.
Now, due to the added weight and bulk of the Air2 and since my diving is a little erradict while I'm looking all over for fish and sharks, the Air2 does become a little unwieldy. To cure this I put one of those magnet holders and a second left should D ring which now keeps it secure and in place, held tight to my chest. I added a second D ring only because my main D ring is lowered all the way down and holds my naked SPG. Stringer on my left waist D ring. Nevertheless, this is probably just an issue isolated to my diving. Cruising along the reef wouldn't be much of a concern.
As far as breathing and using the Air2 goes, I've found it to be quite simple. It breaths remarkable well, but this may be due to the 5th gen design. It has the dive/predive switch so I imagine the benefit here is increased gas flow when set to dive, yet no free flows when set to predive. Very similar to many standard second stages.
For ascents it doesn't present any issues for me per se'. I've got in the habit of always ascending while holding the Air2 in front of my face as if I was breathing on it, this also give a clear view of my wrist computer. I consider it dumping practice with every ascent despite not actually breathing from it. I've had no trouble controlling my ascent and dumping gas. Of course trim is important in this regard. You can't have your bubble above the Air2 dump. But that is no different than a standard dump.
Anyway, I do agree with Tobin that it really has only one purpose... One less hose/standard reg, which could be beneficial depending on your diving. The other arguments against it are rather weak. As far as using it with a BP/W... Makes no difference. It will work just fine.