I don't know why people are so insistent on believing *theories* they read on the internet. I suggest people put it into effect themselves before blindly saying 2.5" is too big.
Not an internet theory in my experience from my own builds, and not from what I have learned from custom exhaust/engine builders. I have had the same results on both the 22re and 3.0 engines when running an open exhaust. Great for high end rpms, but at a loss of low end torque. A few years back when I installed headers on my 3.0 after a top end rebuild, I went w/an open 2.5" exhaust system, using a hi-flow Mangaflow exhaust and Downey's hi-flow muffler, you could actually see through the muffler. I had almost no low end torque, and had to leave at 3k rpms from a stop. On the highway I had to be deeper into the throttle just to maintain cruising speed of 70mph, any rise in the highway at 70/5th gear required a downshift, the rpm's at 70 were 2700 instead of 3500, my mileage dropped from 17(combined highway/city), to 13-14, and my trail mileage dropped from 11-13 to 7. The only benefit I had from the open exhaust was on the high end. I could easily hit 70mph and still pulling in 3rd gear and w/o the engine screaming, whereas w/the stock set-up it was about 55-60 mph and it would flatten out. It was fun to drive at the higher rpm's, but the loss in mpg's wasn't worth it. I went in and talked to some of the local custom exhaust builders and they all told me the same thing, I didn't have enough back pressure for the lower rpm's and that I either needed to run a smaller diameter pipe throughout, change the muffler, or I could try running a smaller diameter pipe from the muffler back(as they do on their race exhausts). Seeing how I didn't care for the drone of the hi-flow muffler, and that both shops wanted more for a bent section of smaller tailpipe than a cheap turbo muffler cost I decided to change the muffler. I went with a 3 chamber Turbo muffler and it did just what the shops said it should. I got back my low end torque and mileage both on the highway and the trail, at 70 my rpms went from 2700-3k, and I didn't lose all that much off top end. I can still easily push the engine to 65 in 3rd if/when I want to, but unlike the open system it begins to flatten out after that.
I did, then a few years later necked it down to 2.25" and LOST low end response and power
If you have done all of the supporting mod's i.e., enlarged the cyls., enlarged the valves, port matched, intake upgrades, and advanced the MAF, then I can see why feel you had less performance w/the smaller exhaust then before, however w/an engine in its stock/near stock form a 2.5" tube and open muffler is simply too large.
