You'd only loose power if the 300 jug was taller, and if it was taller by any significant amount, the compression would be so low it probably wouldn't even run. The 300ex cylinder, piston pin height, and head are all identical to the 250x, same bore, and same overall jug height. All the difference in the 300 engine comes from the crank and rod, Honda stroked the crank and shortened the rod to make the original cylinder height work, and keep the overall engine height the same so no frame changes were needed. One neat trick, however, is to put the stock 250X piston in a 300ex, the 250 is domed and the 300 is a flat top. Cheap high compression upgrade!
1985 Tri-Z 250
1985 ATC250R