A good shop will charge you $300-$450 for the 26k mile valve check/adjust, depending on if they have to re-shim or not. If no re-shim, only a valve cover gasket and one other. I just paid $300 for that work recently in AL. In UT it used to cost me $435 or so, with shim changes. On the '12 I skipped the first interval and did it at 52k. All exhuast valves were right on the edge of spec, running tight. On the '15 that I just had done, I had it checked at 28k and they were all nicely in the middle of spec still. YMMV. At 16k, you don't need to do this work. The header and ECU flash won't have any impact on the valve train.