I have FM8, Arturia DX7 V, Dexed, DX 7 Mk1 and a TX-816 though I've never got around to comparing them thoroughly. I have tried loading the same presets from the DX7 Mk1 into Dexed (in Mk1 mode) and had a quick play through them - not a double-blind scientific test!

Some sounds seemed to me to be spot on - the classic presets that I tried seemed perfectly adequate replacements for the DX 7, particularly for live. It is very impressive for free software. Some more custom synthy sounds - sort of PPG/synclavier character - really didn't sound as effective on Dexed and certainly sounded different - there are clearly some limits to its authenticity. Next time I'm in a "compare this with that" mood, I'll try loading those sounds into everything to see what happens.

It seems you need to test the actual sounds you're using to see if they work on whatever substitute you pick, YMMV depending on the nature of the sounds.