Sorry but I think youre wrong on several counts - using the 159/Brera 2.4 would be madness - it would mean that the engine bay will have to be modified - the 159/Brera 2.4 has a different block and different engine mounts - a 159 2.4 to 147 conversion has been done to a 147 but its a lot of unnecessary work. Much better to spend the money on fitting a GTB2056 or 2060 turbo, a bigger intercooler, uprated injectors/pump, a remap and water/meth injection, with that lot 300bhp is achievable and rather than waste time with the cr*ppy GM F40 box, it would be best to bin it and fit the Fiat 6 speed unit from the 1.9, the driveshafts and DMF can handle the power and torque and are much more readily available. And the gear change from the Fiat box is much nicer than the GM unit's.
The OEM intercooler is already front mounted (in fact ALL intercoolers on ALL cars are frount mounted), a larger one is what is needed. An EGR delete and de-swirled manifold will make very little difference to the power, although a de-swirl will stop any failures.
Agree with most of what you say there, using 159 engine would be madness. 156 20v engine is stronger than a 159 20v engine anyway - they don't crack heads for starters.
I'd stick with the F40 though, as nasty as the gearchange is, the 159 clutch & flywheel will fit, which has been proven by others on here (JBSmith being one) to handle silly numbers reasonably well. The 6-speed box as fitted to the 156/147/GT 1.9 diesel could fit the 20v (I've got one in my 10v) but the clutch situation is always going to be an issue, and it does poke out into the NSF wheel area by quite a bit when fitted on a 5 cylinder.
2.4 20v driveshafts are known for being a weakspot but I'd rather have weak inner CV joints than a weak clutch, and getting something better made up shouldn't be too difficult given that loads of cars use the F40 box. Wonder what Saab used? Perhaps using their inners and the Alfa outers could work, there is nothing wrong with the shaft itself.
240hp was as far as I got with my 20v (hybrid turbo, remap, pre-cat delete, few other bits) and I still didn't enjoy it, after getting there I realised no amount of power would have made me enjoy the car more so I went back to a 10v, which even at 207hp I prefer.