Ok , this has me curious, from what I've read in the manual(21 and 22).. the variable geometry turbo kicks in on cold start ups. Not the exhaust break.. if you manually turn on the exhaust break it will also help warm the vehicle.. this is on the 21 and 22s.. I don't know about the 19 and 20s. . The exhaust break also helped my 18 warm up..
I've done this from day one on the 21 . It's noticeable on single digit nights when my sits out 8 + hrs while I'm at work.. I start the truck ,click the eb button, turn on the cruise , hit the - and it'll high idle at 1000 rpms. Once it warms some hit the + on the cruise and it'll jump the rpms up more... yes, if you do nothing the rpms will go up eventually.. all I can say is try it.