Here's a little comparison on the Toyota AE82 Twin Cam GT from Sushi Express rally team : checking with PerfExpert the difference in torque and power between the original plenum and open velocity stacks.

To make this comparison reliable it was necessary to proceed as following :
- Always stop the car at the same mark on the road before pressing the START button,
- Always WOT at the same mark on the road,
- Perform all measurements in the same direction of travel,
- Be sure to follow the PerfExpert measurement procedures (phone calibration performed, impeccable phone mounting, weak wind, free road, lift windows...)
- If the car remained parked for a while then drive it a few minutes at medium speed to cool down the engine bay. This is to always similar engine bay temperature for all measurements, 
- For each setup (plenum or open stacks), perform at least two measurements having less than 2% deviation to confirm their reliability. Indeed non-repeatable measurements (differences of more than 2%) could reflect a poor phone fixation or poor conditions (road, wind, engine bay temperature...).

Here are the results with this setup :
- 159.6hp @ 7300rpm / 131.8Ft.Lb @ 5500rpm
- 159.1hp @ 7300rpm / 133.2Ft.Lb @ 5300rpm

Here are the results with this setup :
- 150.4hp @ 7200rpm / 128.2Ft.Lb @ 5200rpm
- 151.3hp @ 7200rpm / 130.4Ft.Lb @ 5200rpm

This shows a clear advantage of open stacks in power (more than 5%), and a slight advantage in Torque. But the most interesting is to see the curves comparison :

Note that with the plenum there is a little more torque under 5000rpm and less torque passed this rpm. So really, even if the open stacks give more torque maximum value, the plenum give a more torquey engine at low rpm.
A pity that we didn't have a 100% stock plenum (whose stacks are not cut) and the original airbox + filter. The comparison would have been even more interesting.

If you have questions, please leave a comment!