Could be a 26% boost compared to the club jerseys that fit like tents.

That is, compared to the measured level of wattage required to overcome the aerodynamic resistance of a tent-like jersey (3% of the total watts being generated, for example), the slipperier Pactimo jersey might indeed give a 26% "boost" (I.e., reduction in the watts needed to overcome the aerodynamic resistance of the jersey).

A boost of 0.26 x 3 watts would mean a reduction of 0.78 watts. (Please check my math. I was an English major.)