Like the title says, after I installed the midpipe my car feels slower, especially at the lower range. The sound is indeed identintal to the one from the coupe. The midpipe has only 14k miles on it and is not clogged or anything, I checked all that. It's in as good condition as it's possible.
Do I need to reset the ECU for a simple mod like this or it's normal to lose so lower end power? How can I reset the ECU?

less backpressure?

It shouldn't cause so much loss of back pressure to lose power. If anything, it should give you a little more from more flow. Try reseting the ecu (search the forum for instructions).

You might be a little slower off the line from a drop in back pressure but I don't think a coupe mid pipe will change things enough to notice.

RESET YOUR ECU!! When I replaced my midpipe I felt a loss of power when I left the shop, as soon as I got home I reset the ecu and the power was restored. In fact I felt a bit of a difference in performance.

I will definetely reset the ECU. I read a lot of mixed opinions about whether or not it works and I can see how some people might imagine things since I used to be in the same boat. I am ashamed to say it but I thought my 98 corolla was once fast when I put an intake on it.
I will still reset it since it's never been reset and hope for an imporovement. Thanks for all the replies, guys.

if anything the midpipe will reduce weight so any loss in back pressure should balance out

The midpipe may seem like a huge weight reduction when you try to hold them in your hands, but its actually a very small reduction in the overall weight of the car. Not enough to give you any gains. However the loss of back pressure may make you a bit slower off the line and give you a bit more on top end.

Good luck w/ the reset..hope that helps you.

BTW - the sound is not indeed even close to being identical to the coupe...deeper, yes, but doesn't make the sedan sound like the coupe.

Exactly how does changing the midpipe supposed to do anything other than change the sound?

I don't know what the inside of the factory sedan midpipes look like but my guess it that coupe midpipe should be less restrictive based on size and weight.

That being said I put a coupe midpipe on 2 sedans in the last few months and both cars felt a tad quicker immediately w/o resetting the ECU. Definately not slower.

The install is so easy I'd put the original mid pipe back on to see if it feels faster. Maybe run it up against a freinds car now and after you swap the pipe to see how your car compares to another vehicle with the different midpipes. - just a suggestion

didnt have any issues with mine. Maybe my ECU is special

I didn't have any issues with mine. It sounds deeper than it was but not like the coupe. There is more resonance in the car when driving in too high of a gear for your given speed. If I ever get around to it, I might throw up a sound clip.

I agree, it gets too loud when driving at around 1500-1800 rpm range, it bugs me kinda.

It's probably because you hear more noise but you are not going any faster than before. More sounds gives the illusion that its faster when you dont feel faster. This happened to me too, but this feeling goes away after a while.