Opening up the air flow generally means you lose some low RPM torque, but increase the top end. The stack help maintain the low RPM without restricting the top end.
The air cleaner set-up on my Vitesse is standard. Though it has gone through may changes over the years.
On Mk1/2 GT6/Vitesse fitted with free low air cleaners the standard needles with stronger springs work well. The Mk3 GT6 has a limited adjustment on the mixture and a lack of suitable richer needles, so not so easy to get right.
But my TR7 uses K & N's with stacks, richer needles and a better exhaust, Standard Stainless Steel, which would well, it wakes the engine up. As it's a crossflow engine there is no exhaust under the carbs giving heat problems and the standard air cleaner appears to be very restricted.