Dave, I installed louvers as per plans and later regretted it...my CHT's were plenty low, so I had added that extra drag for little benefit. I went back (after paint, unfortunately), drilled out those louvers, and installed blanking plates.
I've flown with the plates in place for a year, and found CHT's to be satisfactory. The one scenario that demands care is a short fuel stop in hot summer weather....the ensuing climb-out demands a higher climb speeds than usual to keep CHT's under 400F. This is not an inconvenience to me, and now I'm trying to figure out now how to patch those holes back up permanently without creating too much of a repaint job.
I did several other things on the cowl that I think have helped with cooling and minimize drag. For one, I removed that center support that is off-center, creating turbulence in the exit,
and built a replacement with the center support centered and nicely faired into the nose gear in flight.
I left the cowl full length, so it extends an inch or so behind the firewall, and I used baffle seal to seal the gaps around the nose gear. I think attention to details in this area pays off.
I would suggest to builders that they at least install the louvers with screws and nutplates, instead of rivets, so they could be more easily removed.