My .02 cents:
I have the 2.5 in my '99 Cougar MTX, and I felt that the RPM's dropped too slow. It sort of "hung" around 3.5k for a few seconds, so I put the TH Fix in. The RPM's dropped like a stone, and I thought that the throttle responce was better, so I kept it in for a while. After a few months I noticed that I would get a puff of black smoke from the tail when I would stomp on the gas, especially in high RPM's. Also I would get the fuel sender circuit failure code on my OBD. I didn't make the connection at the time.
A while later I needed to bring the Cougar in for some warranty work (unrelated), so I figured it was best to remove the TH Fix...I didn't want the dealer to find some code and deny any work...better to be safe than sorry. The first thing I noticed the moment I took my TH Fix out was that I would slam into the fuel cut off, almost every gear. I was reaching it much faster than I did before. Obviously the TH Fix was some how slowing my acceleration down. Also my fuel sender circuit failure code went away, and the next day the black puffs stopped. So I left the TH fix out for a while. I explained what happened to my friend who also has a MTX 2.5 Cougar, so we did a few runs to see if there was any difference. My Cougar was quicker than his was.
So far it has been almost a year since I took my TH Fix out, and I am MUCH happier now. No problems, runs smooth. My friend also took his out and felt much better about it.
Just my experience.
