Jumping in a bit late to this thread.
I have pulled some wort for an English Barleywine and boiled it down to caramelize the wort but never boiled it down to a syrup before adding it back to the kettle. Did not notice a lot of difference but it was worth a try.
When I make my Wee Heavy, I boil it for two hours which does tend caramelize the wort a bit and I like the results I get from this. The beer has a bit more body and a bit more sweetness than when I was only boiling it for an hour or so.