DrawText is more expensive than DrawObject, but you shouldn't assume that you should avoid it all costs. It's certainly feasible to do DrawTexts on every frame, even in an app with animation. I would use DrawText initially and determine based on performance whether you need to optimize it.
I need to render about 30 words every 3 seconds. What's the threshold of "too many" drawText calls?
You can't mix the two, unfortunately. If you have static text, you could draw it to a bitmap once, then draw that bitmap to the screen on subsequent updates.
Has that been tested, that they are absolutely, positively "immiscible"?
I mean, i was so sure that one cannot show a roMessageDialog on top of a roScreen till dev42 showed us that "yes, you can". And recently i got the feeling that most ro-components were (or should have been) implemented internally via roImageCanvas.