Would sous vide-ing tomatoes result in a deeply caramelised tomato sauce?
It would not caramelize for sure, as caramelization occurs between 110 and 180 degrees celsius depending on the particular sugar - well over the boiling point of water, which is your maximum sous-vide temperature.

However, it would serve a few purposes that might well work.  For one, it would allow the slow breakdown of starches into sugars, just as other slow-cook methods do.  It would also enable your other flavors (spices, other vegetables, etc.) to fully dissolve into the sauce.  This could be useful, and would be one of the major benefits of slow-cooked tomato sauces.  This recipe on Andrew Zimmern's blog is a great example of how to do it, and why;  it includes some good tips for technique, including the very important step of sauteing the root veggies before adding them to the sous-vide bag.

You'd still want to do a high-heat cook at the very end (just like you do with almost anything from a sous-vide) to get that caramelization if that's your goal, but as you note you'd probably need to do that anyway to reduce the sauce if you're going for a thicker sauce (as I often do).  If you're using good sauce tomatoes (romas, for example) you probably won't need to reduce it much if at all, but if you're using more watery tomatoes you may need to anyway.

If you really want to get good caramelization for your tomatoes, I would recommend roasting them in the oven, or even better over a wood fire, first. That's going to get very good caramelization and also get some other good flavors in the tomatoes - a bit of smokiness fits very nicely into many sauce profiles.