Drilling the hole is primarily for allowing air trapped in the system to make it'* way out regardless of thermostat position. If a pocket of air is trapped below the stat, it will not reach temp and open properly, especially after system maintenance. See the coolant fill procedure in Techinfo for more information.
As far as effectively reducing the rating of the stat, it won't. The holes are usually small enough that it really only affects TIME. It takes a little longer to reach the rated temp and open. But the stat will still open at the rated temp, and you can still run (and will run) hotter than that rating when warmed up UNLESS you maintain airflow through the radiator.