In my opinion the best and cheapest way to get more power is to sell the N/A and buy a SRT6.

We had a lot of conversions over here in germany. Btw: we have very strict emission controls. 260 HP are possible with an N/A. We've had somebody who did it (before (203 HP) and after (264 HP) protocol on a calibrated and certified dyno). But it was loads of money. There are also compressor conversions on stock N/A M112 engines which gain around 290 HP. Only useful with a MT, because the AT tranny can't handle too much torque 330 Nm (AT) vs 400 Nm (MT). And there are the V8 conversions. But all of these conversions eat more money than a change from a N/A to a SRT6.

So, buy a SRT6, or take a look at other cars.

Regards