Here's a well written article on how time and temp relate to texture, juiciness, bag juices, gelling and so forth for chicken.
http://www.seriouseats.com/2015/07/t...en-breast.html
I ice bath the bags to bring the temp down, then open them and thoroughly dry off the meat before searing. Two things happen here: 1) you bring the temperature of the meat down so that when it heats up due to the sear, you don't go past your original target temp and over cook the meat. 2) if you don't dry them, you aren't searing, you're steaming them (which continues the cooking) until the moisture evaporates, then once that happens you get a sear.
Past that, make sure your grill is blazing hot, sear as quickly as possible as to not heat up the meat too much.
Oh, and save those bag juices, they are great for making sauces once they are strained. It's basically concentrated stock.