The Castelli Body Paint 4.X Speed Suit is the fastest the brand currently makes, developed for WorldTour use. It's only on sale to the public as UCI insists racing tech be made commercially available, to create a level pro playing field.
Being a serious time trialling suit, the Body Paint 4.X is primed to deliver marginal gains at speeds of 28mph/45kmph and above only.
Castelli has made sure in the course of two years of development, during which they've conducted wind tunnel testing and hundreds of hours of CFD analysis, that cyclists capable of achieving such speeds will indeed notice the difference. The brand's WorldTour level athletes certainly have.
Features of the suit include a high performance Kiss Air² seat pad, extra-long legs with flat endings held in place by flat grip tape, long sleeves with grip tape to stop them riding up and a cut primed to achieve best results in the extreme riding positions common to TT. There's also a breathable mesh race number window to reduce drag, with the whole suit being made from slippery material with as few seams as possible.
Testing at the Norwegian Technical University has proven the Body Paint 4.X suit to be the fastest that Castelli has ever made. All you have to decide is whether you're racing at a level to merit it. If the answer is yes, it will unleash your full competitive time trialling potential.