hi simon,
to switch to encore wont make any difference on the subtitles.
subtitle are 2bit RLE-grafics, like button-overlays as well, so you wont get any better result.
besides that, to create a STL from FCP XML is possible with annotation edit from
http://www.zeitanker.com
encore has it´s own text-file-structure that could also been created with it.
cheers
danny