I would drill AND install the baffles before any coating. Some companies do both inside and out with the ceramic. If the pipes have never been used I wouls pay extra to have the inside coated as well this will insulate the exterior and keep the exhaust temp higher . Very small performace increase. ..Just ceramic coated mine last night and then baked them in my oven at 550 degrees for 1.5 hrs. They are hard as rock now .. will post a pic when I can find my camera . ...Mark...

 

I'll have to make a couple calls and see about cost. Thanks for the tips!