In the early days of planting, an under-soil heating system does show some benefits.. the same techniques are used in terrestrial propagation widely throughout the horticultural industry.
however, I question the use after the first few weeks of planting. Once the root systems are established in terrestrial systems, heat actually can impair top growth, as it forces the plant to put more energy into the root system.
The other factor that needs to be considered, is that our aim in aquatic planting, has to be to recreate the biotope most preferred by the plants, therefore, its natural environment. We spend huge amounts of time fiddling with factors such as KH, PH, ferts in the water column etc. In a natural system, the substrate would actually be COOLER than the water column by a fractional amount.. surely by heating the substrate on a permanent basis will be providing an UN-favourable parameter, as any plant simply wouldn't be evolved to handle this?
Its an interesting subject though, and yes Peter, one that sets experts off against each other very regularly!
What we need, is someone to do fully controlled tests on the method, same waters, same substrates, same species of plants, one not heated, one fully heated, one heated for the first 2 weeks or so. Would be interesting to see, and highly desirable to put it to bed.