changed my spark plugs today! my first real car-job ever, and it was very fun! not a single pause. only problems i had was getting some screws out from the airbox, and broke one inside the hole

luckily it wasnt a headbolt, lol.
took the car for a tour now, and this is what i noticed:
better/faster colder and warm starts. more bottom-end power.
no stutter/hesitation when i give full boost and go from 0-60. better throttleresponse. and is it more silent? perhaps im hearing wrong, but it feels a bit more silent.
i pulled out some old denso plugs, which were pretty white on the electrode tips.
my car ran with a clogged fuel and air filter for a while, so thay might explain it.
two old neighbours looked at the plugs, (one is an old subaru mech/salesman) and said they looked fine. i thought so too. no sticky black stuff on em and they were all dry and clean.
replaced em with some NGK BKR6EVX.
some questions if anybody got time:
i fear my 02 sensor is blown, due to bad mixture too long cuz of clogged filters and old plugs. do you think i should replace it?
and should i give my ecu a reset after running on bad mix for perhaps a year? or will it learn it again now?
and more interesting, when i bought the car i drove 400 miles home. and the car was more aggressive then than it is now. even with shitty filters, probably bad oil and plugs too. my boost has always been good, always going straight to 15 psi.
can this theoretically be solved if i replace my o2 sensor?
my exhaust is also full of leaks. ecu-reset and o2 sensor, guys
sorry for the messy ending here, hehehe.
my next jobs will be cleaning IAC, sealing my coolanttank leak and fix the exhaust.
. i will try and take a pic of the car soon
have a nice day folks