Good questions, proz07.
This whole project started in July when I decided I wanted to block the rear coolant line and bypass the fast idle cam on the throttle body. As I dug deeper in the engine bay I saw first-hand how many comprimises Mazda designed into the OEM lower manifold. I was not impressed with how the runners dodged the ACV and EGR passages... who knows what that does to airflow but it can't be good and I'd rather not find out the hard way. Still need to block that rear coolant line, fast idle cam is done though.
Things sort of snowballed from there. I didn't originally plan on making any other changes but then I decided I wasn't too impressed with the OEM fuel rails and plumbing. Main reason to upgrade now is because I'm afraid that if I were to use the OEM secondary fuel rail I'd have to plug the two additional injector holes in the XS LIM and wouldn't be able to un-plug them in the future.
The primary injectors are only moving a few inches upstream, I don't anticipate major problems from moving them. I'm not saying the distance won't be noticeable, but you can adjust the injector phasing in the EMS (not just the pulsewidth but also when the injectors are opened... similar to the difference between coil dwell and ignition timing). I've heard that other people had funny idling and throttle response complaints but I suspect this may have been due to improper injector phasing... either the ECU couldn't adjust it or the tuner might not have been aware that it could make a difference. According to some people there are potential fuel economy and power gains to be had by moving the injectors further upstream due to improved atomization, unfortunately I'm making so many other changes this might be difficult to quantify. I'll definitely post the results here, even if they're bad.
I'm not sure if I'll want much more power than the stock twins are capable of, but I'm hoping this fuel system will be capable of supporting whatever I decide to do in the next few years.