300XS, yes there is...

I was not implying that Walter himself designed the new models but the aircraft do exist, the XS is a recognized extra throughout the full sized aerobatics circuit. The history is that one of the US Aerobatics team members I think from memory his name was Phil Knight was not satisfied with the original S so modified it, placing in the motor and changing the fuse materials to composite and enlarging the tail surfaces. This aircraft the EXTRA 300 XS was flown the first time in competition at the 1998 World Aerobatic Championships..

My thought behind modifying to the XS with a 300s would be only to improve the 3d capabilities of the plane while keeping it scale. With the full-sized aircraft having the large surfaces, TOC and unlimited Imac pilots could add the balance tabs and large rudder on the tail-surface and still keep it inside the rules for modifications... Really a greatplanes extra is an entry level model so it wouldn't matter what you did to the tail surfaces, personally I would put the hinge points on 50% of the stab and make them air-foiled. Should improve the 3d performance, for pure IMAC I would leave the tail stock, its a proven design.