I wouldn't lockout the preheat hose if you have one...you might pop the top on the box and ensure the thermostat is working. Like some folks have said, if the car is equipped with a preheater hose it must be good, no holes, etc. Here in MD I just went through my emissions dyno test (passed, but it was close. I'd guess the cat is going after 125K and, umm, a little overboost, excessive fuel, and a leaking turbo oil seal) and they didn't even bother to pop the hood or look under the car.

My 90 7xx doesn't even have a preheater hose.

While you have the air box open, I would recommend stripping out all the foam....it's deteriorated by now anyway and will/can clog the amm screen. Next I would recommend replacing the stock air filter element with amsoil foam air filter...works better, lasts longer, and doesn't restrict airflow....the stock paper element restricts airflow above 4K (although nøt much, roughly halfway to 'replace' on my filterminder gauge). You can also pull the entire box and remove the venturi/restrictor in the air hørn that gozinta the fender. If yøu've broken the little tits on the box that the clips that hold the top on snap into, the clips themselves can be replaced with stainless steel clips that are screwed (and some belt and suspenders types also epoxy/screw) onto the box.

Easiest way to increased performance on a turbo is to increase the boost...either by adjusting the wastegate (may not give you enough) or by bleeding the boost signal to the wastegate with a G-valve or somesuch. See turbobricks site for info. I wouldn't increase the boost without improving fuel flow, use a rising rate fuel pressure regulator, SPG9.com sells them for both saab and volvo.