I have a 1999 2.3. Last week I had 3 Dinan modifications installed:
-Software upload
-Cold Air Intake
-Free Flow Exhaust.

Before I brought the car to Dinan I ran some tests with my G-Tech. I did some 0-60 and horsepower tests.
Now, this is my every day car. So, I didnt want to burn out my clutch. Therefore I wasnt as aggressive as I could have been. You know what I mean. I didnt rev up the engine and drop the clutch. I did remember to disconnect the ASC though. But, most importantly I feel I was consistent through all of the tests.

Before Dinan mods:
0-60 7.1 sec.
Horsepower = 150 (this is about right, as I have heard that you should deduct around 15hp thats used by the drive train. The 99 2.3 is rated at 170hp).

After Dinan mods:
0-60 6.8 sec.
Horsepower = 145

I cant figure out the loss of 5 horsepower. The car definitely feels faster. So I believe the .3 sec improvement in the 0-60 time. The G-Tech computes the horsepower by having you enter the weight of the car before you do an acceleration test. The method you use to enter the weight is a little awkward, and I was a few pounds off of what I entered the first time. But I cant see how that can make that big of a difference. Heck, a few extra gallons in the gas tank can make a bigger difference with the weight.

But, there is a noticeable improvement with the performance in the car. The best words I can think of, is freely revving. The engine achieves higher revs quicker. There is also a noticeable difference with engine braking. As an example - If Im approaching a stoplight, say in 3rd gear at 3500rpm and I take my foot of the accelerator the car doesnt slow down quite as quickly as before. Im assuming this is due to the less back pressure in the exhaust manifold. Perhaps is also has something to do with the new computer software.

Under normal driving conditions the Dinan exhaust sounds just like the stock exhaust. Under hard acceleration, its a little louder and deeper. I wasnt looking for a louder exhaust. So this worked out fine.

On the downside, there is that darn whistle coming from the CAI. Its only apparent under acceleration. But its a little annoying.
Also, there is the look of the Dinan exhaust. While its smaller (thank God) than the coffee can exhaust that you see on a lot of Japanese cars, it not as nice as the twin chrome tipped stock mufflers.

Overall, I would have to say it wasnt quite worth the $2,100. If the cost was about half that, then I would say yes, go do it.

Here are a few images

Engine before

Engine with CAI

Stock exhaust

Dinan exhaust