I tried boiling some down to syrup for a gravity boost a year or two ago. The syrup itself was awesome, but the finished cider had a bit of a bitter off taste. I'd definitely follow your plan of using a concentrate rather than a full-on syrup if I did it again, but I've been having luck just using store-bought canned/frozen concentrate lately.
The orchard I usually use just started pressing on Thursday. I hope to pick up a few gallons this afternoon and get my first batch going. I've settled on a nice formula that has been working for me lately to make a nice draft-style cider that has a lot of apple flavor without nearly the sweetness level of commercial versions like Woodchuck or Angry Orchard:
2 gallons fresh juice + 1 can apple concentrate, fermented with D47 or 71B along with pectic enzyme and a pinch of yeast nutrient. After about 7-10 days it is finished, then I cold-crash, keg and force-carbonate. I use 3-4 pints of more of the same fresh juice to backsweeten and then force-carbonate. I've gone from orchard to glass in as little as 10 days. I'm usually bringing growlers of this with me to every Thanksgiving and Christmas get-together I go to, and it goes fast.