I had an 07 2.5i... and we still have an 05 2.5i. (along with my specb)
They are good cars. Between the two cars, the only major thing that was done, was the timing belt at the recommended 100k (this needs to be done), and the cat converters (pricey if not under warranty). Oh and a steering rack, which doesnt really seem to be a known issue with leggies... And that was all on the 05.
Other than that... We've had great luck! Can't complain.
As far as performance goes... I mean, its still a naturally aspirated, heavy, four door, family sedan. You can only do so much within reasonable standard to make it a bit quicker. If you want quicker, just find a 05-07 lgt. There's usually a good amount of them out there. I dont know much about the millenia, but it sounds like they would be equally as quick. Guessing the leggie would out handle the mazda. Just a guess though
Dont use 94 and waste your money. Dont put an intake on it either.