Garret made Volvo specific turbos have the step.
That's true, except for one year: 1981 are flat with a gasket, SAAB had a real nice metal gasket on their version, as was typical on a lot of stuff like that they were much more innovative in those days than their volvo counterparts.
Preheat (unless you have smog visual) is basically a bad joke if it's intercooled and you don't live somewhere really really cold.
Non-IC and you need to drive in Siberia/Antarctica/Northwest Territories/Greenland, might as well have preheat too. Non IC and basically most of the radiator blocked would be good while you're at it. Otherwise, pre-heating air to then return it to ambient temp on a cold morning/light load at startup (when the pre-heat might actually be of any value) via the intercooler is a basically pointless exercise. If power output isn't of primary concern compared to predictable running in many varied conditions, non-IC can be logical. Many military/aircraft applications with turbos are non-IC (deliberately).
None of the feed/return lines will be in quite the right place. Downpipe moves slightly. But it does flow a lot better and if coated and well supported with some appropriate hardware seems to last. Originally, B21FT cars had a bracket that tied the turbo to the studs in the head as an additional precaution, since they were so concerned with hot hardware and it breaking/supporting all that heating/cooling vibrating heavy stuff.
Sadly, being volvo, they still didn't do a great job of it and until water cooled turbos and they tied the runners together (90+ manifold), the head hardware didn't last all that well as they warped at the head flange, but are less prone to cracking at the turbo flange on the pre-90 version unlike the 90+ which does crack. Definitely helps to open the 90+ up and smooth it out.
Clean lasting/longevity oriented adaptation is a few custom parts (feed, return, hardware at least), some difficulty sourcing a vaguely decent 90+, and a ton of time. Power output difference...if there's a turbine housing, manifold is opened up, exhaust, intercooler, and cylinder head to compliment it a bit. Never did the 90+ on its own, but under hood temps coated opened up 90+ and straight through exhaust, 3" outlet .63 exhaust housing is night and day compared to pre 90. Plenty of time though.
If it isn't opened up properly it's unlikely that there's a particularly significant gain.