T6 paper element air filters are very good, and in fact they outperform just about any alternative except for no filter at all.
Now, you can get very close to the 'no filter' performance with the same clean air as the stock system.
These were designed with the intent of improving top end performance. They did, but there's a difference between theory and practice. They were dyno-tested in both B and C (T6) filter cans and tested on both 32mm and 36mm Zenith carbs.
Tested to 5,500RPM, peak power improved only slightly; a consistent average of 0.5HP.
But mid range torque showed marked improvement. Peak torque at 3,300 improved by 4#/ft on average and the increase continued until it gave that extra half-horse at ~5,250RPM.
This is real improvement in street performance and response; power where it's required and used and noticeable. These are drop-in parts; installed in two or three minutes. There probably never has been a peak half-horse gain with less effort and cost, and that's doesn't count the mid-range improvement.
Those of you with B filter cans also benefit in carb preservation; with additional installation effort, these prevent the air filter canisters from 'sawing' into the carb tops