I disagree. Tobi would beat Deidara in any form imo. He may be able to trick Tobi and have him lose an arm due to an exploding clone feint but in the end, Deidara will probably just get overwhelmed/surprised by Izanagi. Plus it seems unlikely that he would make a clone feint like that w/o knowledge on his abilities.
Itachi can still catch people in a genjutsu temporarily. If we are to assume that Deidara will start C3/C4 off the bat or something, it seems only fair that Itachi would use Amaterasu or Susano'o too (at least healthy Itachi). Plus he can seal his jutsu into other living things (and maybe nonliving things, forgot what the data book said) similar to how he implanted Amaterasu into Sasuke. If his crows can cast genjutsu, than they should be able to cast Tsukuyomi if Itachi uses up the chakra for it and seals it in the crow (as per the databook on transcription sealing).
There's also some others like Pein to think about, but I admit that SOMETIMES he's underrated