I thought the Extra 300S originally had a 300HP engine, hence the "300" in the name. The 330 was modified with a 330HP engine, but 330 was never a factory moniker, it was added by the owners to designate the upgraded engine. If that is the case, according to P. W.'s website then hers would actually be an "Extra 350" (350HP)... hehehe.

The "X" suffix denotes eXperimental, since the upgraded engine was non-FAA-stock-certified. So the correct designation, as far as the manufacturer is concerned, would be Extra 300SX, not XS.

If I am incorrect here please let me know so that I do not pass out bad info.