Realistic is a relative term. BTW, we owned one of these for 9 years, and just gave it to my daughter. 120k+ & still runs well.
But: once you boost the HP, several things are going to become obvious:
The trans, CV joints, suspension, radiator and motor mounts are all tuned (sized) for a 142 HP car. You're going to shorten the life of most of these things dramatically when you "get it on."
Driveability will suffer, as you push the HP peak up the RPM band. You'll be barking it up to get off the line, as the torque curve will shift up also. Running at higher RPM will shorten the accessory life as well (water pump, alternator, etc.)
Gas mileage around town will suffer as well.
The brakes are sized for a 142 HP car as well. As you zoom about faster, they'll be taking more wear.
I'm not saying that you can't do what you want here; it's just that by the time you do it all and see what you've got, that you might just wish you'd bought a more powerful car instead.
The down-side to hot rodding has always been that practicality suffers, because you're tweaking one side of a system (the entire car being a designed system) without ballancing the other side(s).
Now, I do this kind of stuff myself (well, not with a Mazda...) so go for it if you wanna, dude. Just don't misunderstand what you'll wind up with. The TCO (total cost of ownership) will go up more that just the cost of the modifications.