jasons

The standard intercooler is not bad in its size or cooling capabilities, but like the VW golfs etc, its been put in the most inefficiant of places, in the wing!
Extra cooling after the turbo will lead to more dense air and more power.
The 8/10v JTD's air temp sensor was integrated in the air mass meter, the 16/20v was integrated into the map sensor. - Hope that clears that one up.
Rather than fit another in line IC, you would be better off ditching the IC in the wing and fitting a FMIC, such an an IC from a saab 900 turbo NG 94-98, this will fit in the bumper of a 156 with just a few mods, front mount so loads of cold air.
Now moving to FMIC is only half of it, the real benifit is you now have loads of room to take a straight pipe (intake) from the turbo and fit a decent filter in the inside wing away from the heat of the engine with its own cold air supply through the front bumper.
If your going for a remap, then go for a larger (an 80mm for 2.4) intake with larger air mass body for your standard air mass sensor to sit in.
if you have an 8or10v, then change the map sensor for a 3 or 3.5 bar sensor, the 16/20v are 3 bar anyway, so you could go 3.5 to run extra boost.
the 3.5bar sensor will need chipping in as it works from 1v base, rather than 2v base, the larger air mass will need a little chipping to spool up quicker, but it will get the throttle back to something thats linear and progressive if your chipping the thing.
The best intercooler will only get close to atmospheric cooling, to go beyond that you will need water injection, this will allow you to run serious boost levels. If you go for this you may as well run a 50/50% mix water & methonal for its octane, it will help keep the smoking down at high rpm.
Just remember, you need the boost higher up the rev band, let it all out low down and the gearbox and clutch will not last long at all.
If you have the 140 2.4 with the Gt2256, you will be good for 1.9bar boost at 4800rpm.
the turbos on the 1.9's are much smaller, but it is a very tunable engine and a few harware mods should see 200bhp, with drastic mods seeing 250bhp