I'm not aware of any aftermarket manufacturers that produce a throttle body for the Speed 6. I'm pretty sure that it isn't a limitation power-wise.
There is a company that makes higher flow fuel injectors, but I have no recollection of what their name is. This engine doesn't use conventional injectors, as it is direct injected. Again, until you are making big power, they aren't much of a limitation.
Before you do any breathing mods, start with a tuning solution like Cobb (very adaptable to mods), or Hypertech (limited to intake/exhaust). Read around on the various Mazda forums; mazda_6_club.com (remove the underscores), and mazdaspeed_forums.com (again, remove the underscores), before you start doing a bunch of mods. This isn't your typical port injected engine, and needs to be approached differently when it comes to mods. Do your research, there is a very knowledgeable community surrounding these cars.
