FWIW, once I started racing, my HPDE days went to zero. Literally zero.
Mostly for reasons of cost. I dont think there's enough value to justify transport, fluids, entry fees, etc.
I slightly disagree with the ability to test n tune since it's often hard to get in enough 10/10 laps to really know what you got. Factor in the passing zones and less experienced drivers and I just found myself not wanting to participate at all. Like FGuy said, the prep factor is also an issue.
I dont know one person on my 8-10 person race team that does HPDEs so it must be a common conclusion.
DE has its place as it is a great opportunity to learn to drive a car at speed, safely. But racing is quite different.
I rarely do DEs in my car unless I can use it as an actual test day. And that is the same for pretty much ever racer I know. Thankfully there are more than a few groups around here (NJ) where that can be easily accomplished. But with the vast majority that is virtually impossible, especially if you are in anything less than the advanced group.
>>>Limited track time, more than two groups, less than 45 minutes per session, too high a car count, etc.
And why as a racer would you take their car on track if you weren't working on something specific? All that does is waste time, money, and equipment. You show up on race weekend, do a short practice, make adjustments, qualify, make adjustments, race, do it again the next day, and then go home. (This is why Jack is known as a sandbagger -- tooling around is pointless.

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Taken from an article my friend John Paton wrote in Porcheforus:
For the DE participant considering expanding into Club Racing, there are a number of factors to be considered. First and maybe foremost, Club Racing is very hard on egos (not that you’d expect to find egos in a bunch of Type A’s thrown on the track to battle for supremacy). Club Racing throws away the order and hierarchy of Drivers Education. DE has the comfort of the system – knowing that if we put in the time and effort to work on our driving, our skill progression will be recognized as we move up through the color ranks, and possibly become an instructor. Club Racing throws all this out the window in favor of the decidedly dispassionate race clock. Suddenly, years of driving experience and honing one’s craft are completely ignored. All that is recognized is who gets to the checkered flag first. A Green/Yellow driver can humiliate a Red/Black pilot with impunity. (Race cars are grouped into classes of similarly capable models, usually by some form of power-to-weight ratio, so it makes it challenging to explain away the seconds-a-lap differences that invariably separate the cars in class.) This is particularly hard on long time DE participants who went through the process and attained their upper run group status years ago. The idea of being thrown back into the bottom of the barrel to claw their way up to the top again may have very little appeal, especially when there is the very real likelihood of not ever reaching the top step of the Class podium. It’s not that “you can’t teach an old dog new tricks”, it’s more like some old dogs just don’t want to suffer through the indignity of having to learn them!
In addition to raising the intensity and adrenaline a couple of notches, Club Racing brings a new appreciation of the cars themselves. Strict class rules prevent the “horsepower creep” that seems so addictive in DE cars. The whole concept of suspension setup takes on new significance because, unlike engine horsepower upgrades, “handling horsepower” is “free” in all PCA Club Racing Classes. Success in racing forces drivers to become more intimate with messages that their cars are sending them and to adjust their setup accordingly. Fortunately for the Porsche Club Racer, the pits at every event are full of other drivers in similar cars more than willing to assist with information and experiences to help this process. It’s all part of the larger basket of “racecraft”.
Unless you’ve had Roy Chong’s DNA genetically implanted, racecraft will have to be learned exactly the same way that we learned to drive on track in the first place – practice, practice, practice! Expecting to be competitive right out of the gate against equally matched cars in class and drivers with years of racing experience is the same as the “wunderkinds” that show up for their first PCA DE expecting to show their instructors “a thing or two”. Aspiring Club Racers need to get comfortable with the fact that, early on, they are going to get beaten on a regular basis – and to have the best time of their lives doing so! And that’s the point. Club Racing opens the track experience to include a variety of new sensory inputs and mental demands that go well beyond the confines of DE, and in doing so exposes the driver/racer to a whole new learning process. On reflection, maybe (for some) Club Racing is the next logical step in their driving education.