Bill,
The check valve will not hinder performance. It is basically a little ball bearing that you put into the vacuum signal channel for the power valve. You first counterbore the hole with a drill bit that they supply in the kit. Then you set the baseplate on a flat surface and tap the bottom of the counterbore with a Phillips screwdriver and a hammer. The "X" pattern in the bottom of the counterbore will allow the vacuum signal to pass around the ball when the manifold vacuum is trying to pull the ball down. In the event of a backfire, the ball will push up against the gasket and small hole in the main body of the carb and seal the power valve from the backfire pressure. Also another note, the new generation of power valves are made with a different diaphragm material which is more resistant to backfires. The check valve kit works, and in fact, all new Holley carbs have this implemented into them, and have for 3 or 4 years.
A note on the gaskets, if you use non-Holley gaskets, you're asking for trouble. If you buy a kit and they gaskets are made of cork type paper, they won't work long. The Holley kits now have a reusable blue gasket for the metering block and float bowls. They don't stick to the carb like the old brown and black gaskets do. They are also much more resistant to any shrinking or swelling.
Chris