I might try and find a 12cm housing for your Skyline too. If you can't find a 12cm housing locally try KKD, he probably has some. IMO 16cm ones suit 3ltr+ engines or people wanting to extract as much power as possible from the turbo, and neither of these scenarios apply to you.
That said, you can try the 16cm and then look for a 12cm if you're really not pleased with how it spools. Given the sort of spool you're used to though you will probably very well be happy with it, so again think about what your expectations are (how hung up on quick spool are you really). The difference won't be as big as people think either. The 3.5" downpipe/exhaust is good, you're right both pre and post-turbine backpressure should be kept low with that combo. But the 12cm wouldn't be a problem either, especially at 'only' 450hp. If possible I might look at installing some pressure sensors in both the manifold and downpipe to truly find out what's going on when it runs (if a 4" downpipe would benefit), but you'll most likely be ok.
Honestly it sounds like your head is screwed on right and you'll be pleased with the setup given your experience and everything you're saying. That Volvo is badass. As you mention, just brake boost or (shock horror) if you have any conception of a manual gearbox, change down a gear or two so you're put right back into the powerband, and off you go. So again, ask yourself if you are absolutely concerned with getting the quickest spool possible.
Yes, Holsets run what most of us would class as big boost on factory applications so they are designed to be run at higher boost pressures, so that's where they shine. Regardless, they're solid turbos that will do extremely well in any application that is spec'd properly, including delivering quick spool if things are done with that in mind. Yes, they're never going to spool like BorgWarner EFRs do, but no other turbos (Garretts included) have a patch on them either purely because of how light the EFR titanium aluminide turbine wheels are. You'd be silly to seriously compare a Holset to an EFR given their respective price points anyway, but I digress.
At the risk of pissing some people off, I will say this; Holsets are generally used by people who have a budget. That's not a problem in itself, but it often means corners are cut with the build and things are done poorly. The result is the owner then blaming the turbo for being shit when really it's their own fault for not knowing how to implement the turbo in a properly thought out system that allows the turbo to operate the best it can. So when you go searching for examples and results of Holsets being used online, you really have to be careful and take most cases (ones which claim they are laggy, crap, diesel turbos, etc) with a grain of salt. Then there are various companies and people with an agenda who slate Holsets and feed further into the misinformation, but the point is simple: they are truly excellent turbos by any standard when implemented properly, and when you consider how much you can get them for, you're laughing.
Thankfully, there are people out there who have brain cells and have used Holsets properly. This very thread contains evidence of that and is a real goldmine - some awesome builds that show what these turbos are truly capable of when you don't half-arse every other aspect of the build, as well as lots of wonderful advice that really anyone with a turbo should heed.
Anyway, thank you for coming to my ted talk
and has ten of thousands km’s on this power.