Drosera like the rainforest sisters love spag and will suffer or slow growth in peat mixes over spag, as Whimgrinder said it is predominantly for nepenthes, as for perlite leaching, I wash all of my soils before I use them, and via washing have never had an issue with perlite, perhaps your source is polluted, maybe try another source.
Also I grow a couple temperate/tropical drosera in spag such as filiformis, capensis and spathulata because in peat mixes they grow much slower and attain smaller sizes, but I should also mention I am in a tropical climate so the airation helps to cool, their roots, same deal with dionaea where I use a soil mix of 1spag:1peat:1basalt.
However with spag you have to be carful and clean it as not all Sphagnum is actually Sphagnum in stores, I only use spag from NZ because I know its clean, I beleive that Chile spag is supposed to be good to, other sources can be polluted or worse not real spag.
Many utricularia also grow much better in spag over peat, such as U.Caerula, nelumbifolia and longifolia, only a small handful preffer peat, one being uliginosa, another being Nivea.