The 18T has a larger compressor wheel, but the compressor inlet port is also machined out to suit.
However, if you are looking for the 18T spec. as supplied by most Saab tuners for the higher stage upgrades, these come with the 6cm^2 turbine housing (as fitted on the 9000 Aero). The Viggen housing is the 5cm^2 version.
I did dyno it. However, I was playing with the Aquamist Water injection since the turbo change. It was not running quite right. Lots of negative adaptation ~7%. Since then I have been running without water for a few days. It is much better and data from T7 looks promising. I do not think that t7 really likes it. It could also be that I do not fully understand the interaction of the Water-methanol + T7 ion sensing + Intake temp equation to tune it properly. I am injecting at the front of the throttle inlet pipe. (neaest to IC) We moved the temp / pressure sensor closer to the trottle.
If I did it again possibly would not clip the turbine as much. Basically there is not so much immediate boost under 2000 but it just keeps going to the limiter
Second gear is quite fun now Lately we have been doing lots of testing. Nees More Brakes! We have a few more new parts / concepts in the works...
Water injection is usually unsucessfull when you cannot tune for it, or don't have trouble with knock.
In order to make use of water injection, because the "Flame Development Angle" is affected by the water, the timing must be advanced. This does not incurr any additional pumping losses however, since the "Rapid Burning Angle" of the flame does not change.
In order to really use water injection you'd need a very expensive multi-port setup. The stock Trionic intake manifold will send most of the water to the center two cyllinders. The air can easily turn and go to the outer two, but the water mollecules are heavier and will tend to go as straight as possible.
Also to use water injection properly the a/f ratio would need to be leaner than 13:1. Running rich only cools the gasses slightly, only 1/6 as well as water, and running richer exacerbates the condition that causes knock in the first place, which is an excess of unburnt hydrocarbons at the rear of the flamewave.
No idea how T7 Ion Sensing would react to water. It should NOT affect the knocking function, as that is just a frequency filter applied to the current at 6,400 Hz ... and would not be affected significantly by WI. It could even enhance the engine's ability to read knock properly due to the fact that HO- is one of the Ions used for Ion sensing, and having more ions helps the engine pick up smaller levels of knock more easily.
Anyhow, a stock T7 car might "put up" with water injection if it were shut off above 3,500 rpm. If it were knocking below 3,500, any downward adaptation might be countered by the advantages of not-knocking. Above 3,500 rpm the engine runs very rich, and WI would be superfluous.
As for the turbo, Garrett strongly advises against clipped turbines. But if there isn't a larger A/R exhaust housing available for the Viggen style TD04 you might just not have a choice.
Also note that if boost were to build up as quickly as stock, but with an 18T turbo, you would probably surge load the compressor as you would be operating in the surge zone.