had various debates with polypipe on this very subject, tiling onto polypipe.....
you'll find most tilers wont tile directly onto it.
theres two options that we give to our customers.
lay a decoupling mat with an s2 adhesive, and fix tiles with the s2 adhesive, but my preferred way and one that gets specd by our tile adhesive supplier tilemaster is pour tm rapid level 30 or tm levelflex leveller over polypipe, then use tm ulitmate s2 to fix tiles.
fitting deltaboards/wedi/marmox type boards wouldnt help with the heat coming up. nor more ply or hardie cement boards are also another alternative.
Oh here we go, the pearls of wisedom from the ex-used car saleman now selling tiles!! Mate nobody cares what you think, we're interested in Heating not some crappy tiles.