i was quoting figures from Serpents own site, as distributors of this product i don't think they'd say it has a restricting effect on the engines if it didn't??
Performance
Because of the optimized design, the INS-BOX unit does hardly affect the performance of the engine, it only reduces the induction noise. The performance was measured in various ways: by taking lap-times (no difference), by using data-logger technology (no difference), and on the test bench (approx. 3% difference). What can be felt is a very little loss of throttle response at low speed. But this will be the same for everybody of course. Running the car with or without the INS-BOX has no influence on the setting of the carburetor, which is proof that the air-flow is not restricted.
Using the INS-BOX has no effect on the engine temperature as it
does not restrict the airflow and the engine is not running leaner.
you can read more on the page in question
HERE 