The great planes extra is a 300S and not a extra 300. It gets pretty tricky.

Now the difference between the S and the XS
The full scale XS has an all carbon fibre fuselage design in comparison than the aliminium/ wood 300's and is remarkable lighter than the 300. A new larger engine, a 330 Lycoming was used instead of the 300hp. In the US some 300s were modified to have the 330 tail at least. These Extras are named EXTRA 300XL (300 XS).


An Extra 300 had the mid mounted wing and smaller rudder with 2 seats while the 300s had basically the same tail group but the wing was made much lower due to probems with sight during full sized aerobatics, also the s was for single seat.

To modify the GPlanes s to an (xs) all you really need to do is modify the tail section, bigger all around also the elevators have counter balances.

The XS features a larger amount of rudder area. This increase in area can be seen by the size of the counterbalance as well as in the chord of the rudder at the trailing edge. Extras have been known for needing a little more rudder area and that's just what the XS has.

Top and side views of the XS
http://www.team-rebel.org/graphics/300XSFuse.jpg
http://www.team-rebel.org/graphics/300XSTop.jpg

Hope this is of help