You wont get anywhere near 50whp out of tuning alone.
But yes, if you KNOW WHAT YOU ARE DOING you can use a laptop, a $170 cable, a wideband A/F setup, and pick up @20hp, and an extra 1 maybe 1.5mpg.
50hp is going to take a downpipe and tune.
There are base maps out there, but none for your car yet, and it will likely be a while until there are 08WRX base maps (which is what your 09 FXT would use) out there. Even then, you are putting your faith in a base map made on someone elses car, with limited testing, and an unknown skill level. Now dont get me wrong, there are some VERY good people with some VERY good "off the shelf" base maps. But without knowing who is who, its entirely possible that you will spend some time scratching your head.
As far as you yourself making the changes, you really need to know what you are doing if you want to make the changes without potentially breaking things (well, breaking pistons). You also need proper gauges, Boost and wideband AF are the minimum, EGT wouldnt be a bad idea either.
If you can find a base map made by someone good, then all it comes down to is installing the program on your laptop, getting the cable, downloading the definitions and flashing the map onto your car. Then watch your boost gauge or datalogs to make sure you arent crazy overboosting.
Which is why many people opt for the AP. Plug it in, punch a couple buttons, there ya go. It will function as a boost gauge, and although I HIGHLY recommend a boost gauge on ANY turbocharged car the minute you mod it AT ALL, you can at least plug it in, and look at your boost levels.
The other problem will be is you are in Jamaica. 99% of the open source guys are in the US, so getting ECU definitions for your vehicle will be tough, Its possible that you would use the same as the EDM (european model) FXT, so if there are EDM 08 WRX base maps out there, you may be in luck. But again, there arent yet. This doesnt mean there wont be, just not now, and likely, it will be a while, as the USDM cars get priority (since the big OS guys are in the US)
I dont even know if the AP is workable for your FXT yet either :(
Can you find out if your FXT uses a USDM spec ECU/emissions/engine?