I loved the sound when I had my S. FWIW, I did a track day last year with a friend in his S2k. Same year, both stock except his had a K&N FIPK and I was alternating between lid on and lid off. We were always equally matched on the straight, whether lid on or off, and surprisingly I didn't notice a decrease in peak straight speeds with lid off either. So although I'm sure it makes less power, the difference (when not stuck in traffic) appears to be pretty minimal.

If I had kept the car longer, I would have sprung for the FIPK.

 

I also took the lid off the first month I had my car haha. It sounded amazing, but it was also the last time I clearly heard the intake sound alone. I installed my k&n FIPK intake paired with the invidia q300 exhaust at the same time. I can only hear the exhaust now

 

I have the comptech intake and once you use weatherstripping on the lid to fix the buzz you get the great sound and lower IAT's than stock.

 

I tested back to back with lid on/off. Lost about 15whp.
Project S2000: Part 20 - Stock Air Box Modifications And Not Going Faster

 

I think on a dyno the effect is worse though. While moving there will be more air circulating in the engine bay so it is not as hot and heat-soaked. It was definitely not a 15whp difference when I was on the track and alternating between lid on/off and chasing a friend's AP1 w/FIPK.

 

You are absolutely correct that dynos will not be accurate because you don't have a wall of high pressure air pushing into the car. That said, a WOT intake is sucking a lot of air and with direct access to that crispy over-engine air, it will pull that in to a certain extent even while moving.

Back in the day a few of us did roll-ons on the highway and between lid on and lid off, there wasn't a noticeable change. However, the improved throttle response between either and the K&N FIPK was tremendous. The FIPK was much more immediate.

If you want some nice intake sound, the FIPK at least will give you that and better throttle response.

 

I believe the 'dyno' testing in that article was based on a dyno simulator, driving the car on the road and measuring its stats, and extrapolating hp & tq. So it wasn't stagnant dyno air.

 

It was done in real world conditions and the car saw a significant decrease to acceleration with the lid off. 3rd gear pulls from 3700-7800 increased in time by ~1.4 seconds. He used a spreadsheet calculator to determine that 15whp at the upper end and 5-10 lb/ft torque at the wheels across the ranges tested.

 

There were so many flaws in this test. A guy putting his foot down on a country road is not an accurate test.

 

I used the same stretch of road, same direction 4 runs, 2 with the lid, 2 without the lid, all within 5 minutes total. If you looked at the data, the data was very repeatable with and without lid.