Hi

    I have had quite a bit of experience with vortex generators and it is a rare case that the VG's do not  slow down the airplane. They are very good for improving design problems noted in flight test but there is always a cost in addition to the time to install properly. The chairman of the board once said "rarely do we need to go slower, people buy airplanes to go faster". The best way to check this is to measure passing times.  I have done this with a number of airplanes and the result is slower.
    You can do its your self, the technique is to get the base airplane (it requires at least 2 a/c) and then do your passing times.  This is done by first making only one change at a time (the test change) after the base line run. 
     The passing time itself is done by first synching the two propellers (same rpm) by looking through the prop on your airplane.  It is best done at full throttle as this removes any adjustment errors but a lesser power setting can be used if the power is set very accurately and the exact same power used for the performance change test. This must be done in smooth air. Then after determining which is faster, the faster airplane starts from behind and using some point on the wing exactly, measure with a stopwatch the time to go from the tail to the tip of the spinner of the slow airplane.  This can be converted to Feet per second by dividing the length from spinner to tip of the tail by the time. 
    I have many hours doing passing times and lots of valuable info can be obtained and sometimes the result is very suprising.
    If you have questions don't hesitate to contact me.

Jack Webb
L 360, L IV
ATP, CFI Airplanes & Instruments, Multi, & Sea
Many Type Ratings
Experienced Experimental Test Pilot both multi and single engine
Former Chief Engineering Test Pilot for a Major Manufacturer
AeroSpace Engineer BSAE
Ohio