first engine problems
 

Ok so heres my history:

2004 MT GT 78k Original Engine

Its been in my ownership for the past 20k miles, 3k oil changes and about 5k miles ago I changed the spark / ignition coils just because I did not know how long its been since they were last done. Recently replaced rear O2 sensor because of a CEL. Oh and I used to get 220 or so per tank now only 180.

So here is my problem. Ive had multiple CEL for misfire as of lately, thought maybe I overtightened and damaged my spark plugs when installing them so I took it to a trusted local shop to see what they thought, everything checked out fine. Another thing is that my idle has dropped from a steady 850 900 ish to a varying 750 to 950.

I'm thinking I need to get a compression test done because I hate to say that it my be my engine going out the window. The only thing is that I dont have that many oil change records and my last few have all been synthetic anyways and I know a mazda dealership could possibly use that against me.

dont know if you need this info but mods are:

K&N typhoon 2
AP underdriven pulley
ACT streetlight flywheel
turboxs catback
turboxs single resonated midpipe.




So my question is, should I take it in and have a compression test done (after swapping back to stock cat) or wait to build up some oil change receipts.