2005 2.5i outback 1st gear bogging and low power

[Azzamatazz]

Hey folks!
First off i'm new here but ive had my 05 Outback for a while now and i love it.
Now first things first, I'm a semi-experianced backyard mechanic, but most of what i know is old 90's and earlier toyota's (inc a 4L v8 Soarer). However this is my first subaru and thus my first flat 4 engine.

Anyway its recently developed an odd problem, Its started bogging and spluttering in 1st. Only happens at low rpm under load, such as takeing off from traffic lights. This happens when the car is cold OR hot but its worse when its cold.
So far i suspect either a fuel pressure issue such as a clogged filter or injector, or some sort of problem with the auto transmission as the problem only manifest in 1st gear, but obviously being an auto its almost impossible to replicate the right conditions even with the manual shift mode.

Anyway i guess my question(s) is has anyone had this same issue and what turned out to be the problem? (its also eating oil but ive been told thats pretty normal for high KM EJ25's)
Also what type of fuel filter do they use, inline or in-tank(sock type etc)?

Oh and another thing, its seems to be a little bit down on power overall but thats most likely due to whatever is causing the spluttering problem.

Final point, what are the best ways to get some extra power out of these for cheap(ish). I do plan on an EJ257 STI turbo engine swap at some point in the future but i would like to see what i can get out of the NA motor first. Once its running right obviously.

Anyway thanks in advance!