although FM11 is not certified for Mavericks by FM Inc. the runtime solutions of FM11 seem to work on Mavericks without problems. So there is no real technical need for the upgrade in my eyes.
Sadly there is no way for runtimes to change the format of a former version automatically to fmp12 format. It will only work, if all binding keys are the same. We already discussed that in details https://fmdev.filemaker.com/message/86703#86703
Therefor people will have to download a FM12 demo just for converting their files to be able to import their data unless you offer this conversion step as a service (which we do). I doubt that this might technically work in any way as an online service.
I also still run v11 on Mavericks, but have experienced problems when typing in text into text fields. The text that you type in does not appear until you click out of the field. I have not really tried the run-time, but I figure if I get this problem with FM11, then I could get the same problem with runtime.
Personally I don´t see such problems on our apps based on FM11v3 on Mavericks.
Sure the runtime will behave the same like the full version, since it´s mainly the same code base.
But I would guess, that there is another problem involved - maybe the font or any utility like Typinator or something like that.