Longer range will depend on a number of things. First off, the antenna with the windsurfer attached is much more directional than the stock configuration. It is easy to lose signal strength by not pointing the RC directly at the P3 and slewing it as appropriate. Range also depends on what the transmitter is trying to punch through. I've found that I can double or triple my range *over water* - which is a best case scenario. If I am sitting in a open field surrounded by 150 foot spruce trees I don't get much more distance than stock. My FPVLR antenna which has about double the gain of the windsurfer helps considerably more - and is even more directional.
Over all, I think the windsurfer gives a noticeable range increase in most situtations but it can be subtle. I am also assuming that you are actually testing correctly by running the same route under the same conditions (ie, same day, nearly same time) and measuring the distance to loss of signal. If you are just going by the seat of your pants, you may well not notice a difference.