Frankly speaking - we have been doing enough enrichment from the time my kids were born that they probably do not need the test prep. We have spend a lot of time with our kids so that they are well rounded in academics, EC, socialization etc.
However, apart from a very enriched environment at home, we also enrolled them in a test prep course. Did that help? I don't really know. Since 2nd grade - my DC2 (who is in HGC) was testing very well. Scores in Raven, TN2, InView were off the charts. Now in 5th grade - scores in Map-M and R are also extremely high. DC2 scores very well in IQ and Raven kinds of tests.
DC2 is not a hard worker though and being in the prep course forced at least a few hours of work every week. DC2 is an extremely good test taker. Top 1 %.
DC1 is a hard worker - also a magnet HS kid - but a poor test taker. Still gets good scores, but is not the top 1% on tests.
I think the prep can help to increase the score somewhat and gives an edge. But it is just that - an edge. It is not the whole enchilada. The content still has to be learned and they also have to be avid readers by themselves to score well in reading comprehension.
I was using some college textbooks (Math 101) to teach Math at home for quite sometime, and that helped too.
Do I recommend the prep classes? Yes. Will that guarantee that someone gets in? I don't think so.