Never hear of this theory. I have seen monster VFTs in 4" pots in semi shade in a greenhouse and my own plants are not exactly small and they have never been in anything smaller that a 12" window box.
I personally do not see any reason that soil heat would induce larger growth. Truth be told I would think it would be more prone to causing minor trauma or sustained stress. Not to mention that adding heat to an perpetually damp system would be really conducive to fungus growth.
If you are interested just try it for yourself on some spare plants. If it works then good, if it doesn't....
I've done this experiment on a few of my plants and so far there are no major improvements in growth, but I've only done this for a few weeks.
Keep in mind that Sarracenia Northwest (aka Cobraplant.com) is located on the coast of Oregon where the growing season is cool and overcast. They mention that the increased soil temps are benificial for their area, but might not be needed for other areas.
i got my most recent flytrap early Spring and kept it inside most of the spring by a sunny window where it stayed around 70 to 75 degrees farenheight and it didn't grow fast at all... i put it outside in this Atlanta heat on pavement in a square copper container that fit the 3" pot tightly (to hold water) so it got full Georgia sun all morning till about 3 o clock in the afternoon and its been hella hot and relentless humidity... the copper would burn a little to hold onto in the afternoon...
i havn't had any problems with fungus and i have never seen so much growth in a flytrap of mine (but i havn't really had many) but in my opinion
i would say it was the soil temp ALONG with the full sun it was getting...
I got all my flytraps from these guys (along with the bulk of my other plants) and have been using the gravel method on my oldest pot of plants since early spring. I can't tell if they're much bigger than other VFTs of similar age, but I do know that they've been growing much faster. It puts out two or three leaves in the time it takes my other VFTs to start one, and they're much stockier and upright in the pot with the gravel. The traps are almost twice the size now as they were when I first got them. And this is all from a pair of plants that were nearly dead at the beginning of the season. At the very least, it doesn't seem to hurt. I am up here in the northwest, however, so I can't speak for how this technique works in other climates.