Here's the deal with a GC2. I know because I own one and use it all the time in the winter.
First the basic unit measures only four things, ball speed, launch angle, azimuth, back spin and side spin.
It is important to understand it measures launch angle and azimuth relative to the direction, both vertical and horizontal, the wide angle lens camera is pointing. To this end the unit has a built in compensation for the unit not being level in all axis. There is a software routine to set the unit up which requires the unit to be perfectly level. It stores the offset values of the accelerometer so when the unit is not level it knows how to compensate. If this is not done the readings will be off.
If the above is done correctly then the unit takes between 7 and 12 pictures per shot (less for fast moving, more for slow moving). These pictures are used to DIRECTLY measure the things listed above by computing the distance, angles and in the case of azimuth size, between the selected pixels and/or size in the images selected.. There is no way to juice these readings PERIOD.
What can be done, if you are using the Foresight range or course simulation software, is a constant multiplier to the carry distance computed. This scaling factor does not in any way affect the actual measurements.
What ever club you specify includes a constant smash factor. It is something like 1.45 for woods and 1.36 for irons. The swing speed displayed in the software is simply the ball speed divided by this number. You can also screw around with the wind speed and direction to effect the carry, ball height during flight, rate of spin change etc.
As to accuracy the things measured are the most accurate in the launch monitor industry. Foresight publishes the accuracy specs of each unit as shipped right on the front page of their website. I would note the radar units do not.
In summary if you hit on a properly maintained unit the results will be the same day to day, club to club within you own reputability. I do it all the time. It is actually more repeatable then going to the range or on the course since you do not have to deal with the uncertain wind, temperature and elevation conditions indoors.
Some people are reported to have trouble repeating their course swing indoors. My experience, after having quite a few people hit on my system is that what they really have trouble with is their ability to know how far they are actually hitting the ball on the course. The indoors/outdoors thing usually goes away after hitting about 10 shots.
There is another add on unit made by Foresight called an HMT which actually MEASURES directly clubhead variables like swing speed, attack angle,etc. with industry leading accuracy. These units require you put a few stickers on the clubface. I have no experience with these units since i do not own one. People that have them seem to think they are the greatest thing since sliced bread. They do output an almost bewildering amount of numbers per shot. These are in addition to those provided by the basic unit.
