The Viggen IC can "handle" plenty of flow. But whether it can handle the T5 engine depends on if you mean the 2.3L or the 2.0. I'll assume you mean the 2.0, in which case it should be "adequate" but it will likely cost you between $300 and $400 to get the Viggen IC and all the hoses. For $600 you could have an intercooler 3 times the size.
The naturally aspirated camshafts will only give about 12-15 hp on a T5 engine if tuned for them. T7 cars had MUCH worse camshafts, but MUCH BETTER inlet ducts. The T7 head is considerably better than the T5, but it just has bad cams.
Ok well .. I should be specific, the T7 doesn't have BAD camshafts. They're just designed for spool up and torque rather than horsepower. This was because Saab revised the swirl characteristics of the inlet ports, which allowed them to make the inlet ports bigger and get more flow, with less aggresive cams. This is why the Viggen, even with worse camshafts, slightly less boost, and a more restrictive Turbine housing makes MORE horses than the equivalent 9000 Aero. The T7 head is that
Something else I should note ... DO NOT
port your T7 head. It flows extremely well and has swirl characteristics that are dependant on the exact shape of the inlet ports. Unless you are out for a pure race car, porting will yield almost no top end improvement, and will substantially increase knock sensitivity at lower revs.
Anyway ... back to the your question Tweek. Yes a Viggen intercooler will work. And yes you need BOTH cams! But it won't help a whole lot. It's been tested to only yield a very very small improvement. Look for an upgraded one / intercooler water spray.
As for the headers, that's a great idea. They will significantly increase spool up, and decrease EGT. If they are made well, with a proper Inconel alloy they should also be just as durable as the stock manifolds. Something else to consider while you have your engine apart is ceramic coated pistons. One of the major factors causing knock is heat soak in the piston.
The piston can only shed heat in small amounts through the oil sprayed at the bottom, and through the piston skirts and rings. This is a relatively long distance for the heat to travel, and because of conductive properties of metals the center top of the piston can get quite hot, as can the exhaust valves. Coating both with ceramic reduces the heat absorbed, and thus not only reduces the likelihood for detonation, but also reduces coolant temps. (Coolant temps also greatly affect detonation)
Putting all those things together (good intercooler, water sprayer, headers, ceramic pistons with moly coated skirts, ceramic coated exhaust valves if possible) and you should have one really really mean Saab. And that's before you even turn up the boost.