The first response came from GM Nigel Davies, who commented on my Facebook page that "It's possible that 5.dxc5 is a VERY good move". Another friend also pointed out that the most recent book to advocate the Tarrasch, Aagard and Ntirlis' 2012 Quality Chess publication, also regards the move as very strong. They in fact prefer it delayed one move, after 4.cxd5 exd5 5.Nf3 Nc6 and only now 6.dxc5.
I have not seen their book, but I assume the reason is that with the black knight already on c6, the line 6...d4 7.Na4 b5 8.cxb6 axb6 is less dangerous for White, because Bd7 is less of an immediate threat. Even so, they presumably consider that Black can hold the balance after 6.dxc5, although I guess he needs to be well-prepared.
The other GM response came from John Nunn, who sent the score of one of his smart tactical wins with the opening (see below). However, he also warned "But really, if White knows what he is doing, the Tarrasch is a bit miserable, as he can always keep the draw in hand". More than any specific variation, this is probably why the Trash is not more popular. In fact, I recall John saying, when he decided to give the opening up, around 1980, that the problem with it was that White seemed to be able to get away with 3-4 small inaccuracies, yet still be drawing, whereas Black only had to commit one, to find himself really suffering.
I do not doubt that this is true at GM level, or even close to it. However, at the ordinary club level, tactics are everything, and the Tarrasch has the great merit of giving free, active piece play. In such positions, few club players are going to be able to control the tactics well enough to maintain a small advantage and keep the draw in hand, so I think John's fears are much less well grounded at that level. John, of course, had to play against the likes of Ulf Andersson (in fact, if my memory does not deceive me, the game which made him decide to give up the Tarrasch was a tortuous afternoon of suffering at the 1980 Phillips & Drew, against Ulfie himself...), so his fears are much more understandable.
However, even GMs of anything below the top rank were liable to get the Nunn treatment in the Trash, as the following display of pyrotechnics shows: