This has been argued over before but I'll just jump on my soapbox once more. For the MS3's price you should not expect a car that can be modded and still prove reliable. Mazda has built the MS3 as a fun, well-finished and fast mode of transportation for UNDER $23,000. Like all cars the MS3 is a system that is engineered around controlled tolerances and failure modes. Adding a bigger turbo may not break things in the short-term but it will severely compromise the long-term durability of an drivetrain that was designed with certain parts that were proven to work relaibly together for specific cycle times.
It is UNREALISTIC to expect Mazda or any manufacturer to build a product at this price and feature set that has been tested with limitless variations in the basic design -or- use parts that are overbuilt to survive any forseeable interaction.
If Mazda were to build a car that could be modded and still reamin as relaible as it was designed the product would cost multiples of it's 23,000 price.
If you want a reliable, fun and well-designed product then the MS3 is a good choice.
If you want to mod the hell out of it then you should look for a older STi or Mitsu EVO. I would argue that those cars are more moddable, but they also cost ~$10,000 more when new and are no where as refined a daily driver as the MS3.
IMO