I actually tested this on my car last fall and have track results. This was the only change that i made on the car besides shock tuning and jetting changes. But i have corrected times to compare the results. I ran at the same track in the same month, but 1 year apart. Track day 1 was 10/6/16 when i was running a 13” collector extension. Track day 2 was exactly 1 year later on 10/6/17 when i made a new 18” extension the same 3-1/2” dia. The car is a 68 charger with a 505 440 based stroker. 10.4:1 comp. with edelbrock rpm heads with std port opening. Just cleaned up and polished with a good comp valve job. Comp cams solid flat cam with .640 and .620” with 262 and 264 @.050”. Victor single plane with a 2 circuit 1050 dom.

The results:
10/6/16
10.817 @128.42. Corrected time

10/6/17
10.575 @129.42 corrected time.

Granted, this is a 4 speed car and the e.t. difference cam be partially attributed to me getting more comfortable powershifting the car. I only get out about twice a year up until now.
Just wanted to add my results for you to see. It looks like the car liked the longer extensions.