I am one of those that learned to fly on a 'hot' Avistar. MDS .48 installed. I think the issue is that a lot of beginners don't want to learn to fly on a plane that can't power itself through a loop, or battle a 15 mph headwind. I've seen some of the other trainers running on .40s. They're a popular setup among the retired generation of pilots, but most of us in the teens to 30s cut our teeth on video games that required lightning fast response times. It might be a great idea for someone to learn on a milder plane, but if they get bored out of their minds they're not likely to come back for another session.

I went from newbie (with only sim experience) to solo in something like 20+ trainer-cord flights. It took about 2-3 months of visits to the field with my trainer. I am far from a prodigy so I would expect that with the proper level of interest that anyone should be able to do nearly the same (plus or minus 5-10 flights).
Once solo, I don't think anyone under the age of 60 is going to remain happy with a .40LA powered flat-bottom large dihedral trainer for very long. Which means two things: 1) they've just spent $250 and 10+ hours assembly for a plane and engine that they've outgrown in 2 months, and 2) they're looking for a second plane and they're likely to 'over-plane' themselves for a second plane before they're ready for something hot.

In fact, this is what I did despite having the Avistar. I tried to run after learning to crawl, skipping the walking part. Which is my contribution to this thread. WARNING: Do NOT be in a rush to fly the 'hot' planes after you've solo'ed. I stepped up to 3D planes way too fast, and cost myself two very nice planes because of it.
This year I've gone back to a trainer, but I also have the experience to know that if I don't have a hot trainer I will lose interest. This year's plane that I've chosen to continue refining my skills on is a Hobbistar .60 with a .61FX. I witnessed a guy train on this combo last year, and it works out well. It might take a bit more care to learn on, but the size is easier to deal with for newbies, and it allows for a lot of spirited flight once you've soloed.

One last point. You can always throttle back, but you can't add more power to a plane that doesn't have it. Most trainers I've seen end up looking a bit ragged, which means there isn't much chance of selling it for anything near what it cost new. So if you power it with a 'hot' engine, you can always migrate that engine to your second plane and write off the cost of the trainer frame as learning expense. What the heck is someone going to do with a .40LA when they move up? Admittedly it's a cheap engine, but why not spend a few dollars more and have one that can power the next airframe?

I know this thread concerns choice of instructors, but I like to think that I've brought up a few warnings/considerations for those that are training, so I think it's relevant.