The 3800 has a pretty extensive aftermarket so there are a lot of options for it depending on the goals of the build.
Generally the most cost-effective route to gain power in any 3800 is to turbocharge it. A turbocharged bone-stock 3800 will make as much power or even more power than most if not all M90 equipped 3800's.
It is pretty easy to mod an M90, but there will be much greater gains by switching to a turbocharger from the beginning.
Before modding you should consider your power and drivability goals for your car
Naturally aspirated engines will make predictable horsepower with minimal heatsoak. Supercharged engines will also make predictable horsepower throughout the rev range, but will suffer from heatsoak. Turbocharged engines will typically make crazy amounts of power once the turbo spools with less heatsoak than a supercharged car. The power output of a turbo is not as consistent across the rev range.