Some new results. So I'm on a single monitor now, should/might give more reliable results.
Haven't compared 50Hz yet, but FAC IV runs buttery smooth on 0.15!
Again: definitely different behavior in 0.16. Though throughout I might get the best smoothness from 0.16, driver vsync On (or Adapative), openMSX vsync On, but must be in fullscreen! Windowed will give me a (small) stutter every now & then.
In chronologous testing order:
1] 0.15, driver vsync Auto (= 'application controlled')
* cpu in the range of 1 to 5%, w/ occasionally spike up to 40%, which will gradually go back to low values again.
* smoothness is good (as I described before: can run smooth for a while, but at a point will have some stutter/judder, and go back to smooth again. Pausing & resuming seem to help when rendering goes juddery.)
2] 0.15, driver vsync Off
* very low (best) cpu values: 0 - 2%
* slightly more chance on a stutter, so it seems.
* FS: getting a thick 'hum'/tearing bar now of about 15 to 20% of the viewport in height, moving (slowly) from top to bottom.
3] 0.15 driver vysync On
* seems to behave same as case 1.
4] 0.16, driver vsync Auto, app (openMSX) vsync Off
* windowed: mixed results, seems more 'stuttery' (more chance) than 0.15 though;
* FS: tearing / small 'hum bars' appearing on alternating horizontal positions
* can be as smooth as 015, but seems overall more problematic (esp. FS)
5] 0.16, driver vsync On, app vsync On (triple buffering also On)
* FS seems perfect, fluid throughout;
* only drawback: perhaps slightly more input lag (should test a shooter for that);
* windowed is less good: a stutter every now & then.
6] 0.15, driver vysnc On, triple buff. On
* very good results, both windowed & FS (latter even better I think)!
So throughout, changing openMSX's vsync doesn't seem to matter a lot, except for what I described at case 5 (combining it with the vsync of the driver)!
(Some of my findings here might be quite specific to my system though (hw in combin. w/ OS & drivers) 
)
Note that I have Threaded optimization Off, when that's on or auto, it will result in (very) high CPU usage! (Again could be a my system issue.)
