17's to 14's is a multi-thousand dollar improvement on any car. It's even worse on an NA car because all the power making is hidden deep inside the engine... there's no turbo control system right at your finger tips to monkey with.
$300 on an NA car will change performance about as much as having a spat of cold weather (which makes the air a little denser). We're talking intake, pulleys, stuff that's almost imperceptable unless you use your imagination. You can also consider doing cosmetic things that make it seem faster and funner, like get a loud muffler, get a short shifter that feels sportier, get some new shocks or sway bar. Oh yeah, in most of those cases $300 is just getting you parts, not install.
Don't get me wrong, it's fun. I did that kind of little stuff to my RS which has the same engine as you, and it makes commuting a little more fun. But, reel in those expectations a bit
EDIT - sorry, I see that you actually plan to put in 1k per year. At that rate you could start after saving a few years. Yeah, cheapest way might be to sell your car and buy a used WRX. Next cheapest might be to swap in a 2.0T, especially if you can do it yourself and you don't mind investing months of free time in it. Next cheapest might be to buy a turbo kit for the EJ25, but keep in mind a) you're going to need to upgrade everything not just strap the turbo on b) EJ25s like to blow headgaskets with not a lot of boost - so you're going to have to dance around that issue big time while trying to get an EJ25 to pull the weight of a Legacy body to 14's. d) most expensive option is probably trying to get to 14's all-motor