Keep in mind that many (most?) VS furnaces adjust airflow to a pre-determined number up to (usually) 0.5" esp (well I shouldn't technically say esp - it will compensate for the filter which is often in the furnace, or at least near it). This alleviates that fear somewhat - until the filter is so dirty it drives the static above 0.5".
Maybe I'll measure the temp/o2/co of the exhaust with the different fan speed settings on my furnace one of these days if I get inspired. I suspect efficiency stays the same irregardless of airflow (within the temp rise variation set forth by the manufacturer).