The quality of the TMIC will play a big role in whether it's a worthwhile investment or not...
Aftermarket TMICs can be evaluated in two ways:
1) Restriction (Pressure Drop). If the pressure drop over the core is 3PSI, to achieve a manifold pressure reading of 18PSI (which is what's measured by the ECU), you would need 21PSI at the compressor. If the pressure drop over the core is 1PSI, to achieve that same 18PSI manifold pressure, you would only need 19PSI at the compressor.
2) Cooling Ability. This is the difference between the temperature of the charge as it enters the TMIC (T-in) and the temperature as it exits the TMIC (T-out).
The whole purpose of an intercooler is to reduce the intake charge coming from the turbocharger. At the end of the day, cooler air is best (T-out), regardless of how it gets there.
A quality TMIC will balance restriction and cooling ability. There are two main factors in what makes a good TMIC:
1) Design
2) Fin Density
There are two types of intercooler design: tube/fin and bar/plate. Generally speaking, tube/fin are more efficient at cooling, at the expense of restriction (vs a comparable bar/plate design). Factory TMICs on Subarus all utilize the tube/fin design, where most aftermarket companies utilize bar/plate.
The whole purpose of designing a quality aftermarket TMIC is that you balance both restriction and cooling ability. By using the less-restrictive bar/plate design (the turbo isn't working as hard, and is therefore producing less heat, reducing T-In value), and utilizing a higher fin density to adequately cool the intake charge, you can achieve better results from the more efficient / more restrictive tube-fin design.
Let's setup an example using some numbers tossed around earlier (and some others to make the equation work)...
TMIC Efficiency = ((Tin - Tout) / (Tin - Tambient))
Manifold Pressure: 18PSI
Tube/Fin: 3 PSI Pressure Drop
Bar/Plate: 1 PSI Pressure Drop
° / PSI (T-in): 10
Ambient Temp: 50°
Tube/Fin Efficiency: 80%
Bar/Plate Efficiency: 75%
Therefore...
Compressor (Tube/Fin): 21 PSI / 210°
Compressor (Bar/Plate): 19 PSI / 190°
Manifold (Tube/Fin): 18 PSI / 82°
Manifold (Bar/Plate): 18 PSI / 75°
Despite the bar/plate being 5% less efficient at cooling, the fact that it started with a 20° lower temperature (from the turbo not having to work so hard), meant that the end-result was a 7° cooler charge once it left the intercooler.
All else equal, let's assume that the bar/plate efficiency was 70%; the outlet temperature would be 92°, which will ultimately mean that it is a performance downgrade.