Now as for the part to increase performance, that’s all depends on your engine size & intake manifold size, it’s why there’s a lot of mix reviews, some swore by it and some feel no power gain…
Example on k24z7, it’s 2400cc per revolution, if your intake manifold volume is smaller than 2400cc (usually stock im will always have less air volume require per revolution) it will require to draw more air from outside of the tb to feed enough air for 2400cc per revolution, the vehicle can still run smoothly even if your engine can only inhale let say 2300cc of air per revolution, your just not going to perform at engine’s full potential..
During wot or force induction engine, the im size will have no effect…
1 way to know if you need tb is this, if your car is manual transmission, during stop and go traffic, when you shift into next gear and gas it kinda hard but not floor it & you feel the car is kinda “choking”, it means it’s better if you have larger plenum intake manifold, and tb spacer might help… not every car needs it, & it’s all depends on your mods, it’s why some people feel no gains & some people feel gains.
will too large intake plenum volume hurt performance ? in most cases, yes... when you have a larger plenum, it now require longer time to replenish a larger im, think like this... when you turn on water hose at full blast, will you fill up a 5 gallon bucket faster or a 7 gallon faster ?