You need to do it with a script, not an expression. With a script, you can use yourTextLayer.sourceRectAtTime() to get the left, top, width, and height of the actual text. Since you are already running a script, that would be the ideal place to put such code, but it could also be done in a second script that you would run after you populate your comps with the new text. That would let you get rid of the sampleImage() expressions, which should save you a ton of render time. It depends on your project set up (and to some extent on how script-friendly it is), but such a script could probably be written and tested in a few hours if you know what you're doing. It would be specific to your project though--it would be tough (I think) to write a general purpose tool for this.
Dan