Tobi is my favourite character, but stating that whoever gets warped into that dimension is put into a genjutsu is very false. first of all, we didn't see them getting put under a genjutsu when sucked in, only coming out in a genjutsu. meaning tobi could have put them under at any time. secondly, neither sasuke, karin, kakashi, or naruto's clone were put under a genjutsu when warped. meaning tobi has to go into his dimension and do it manually. and minato can attack him when he materialises in there.
tobi trying to warp minato does not mean it would have worked. oro tried to summon the fourth, it failed. hiruzen tried to block it, it failed also. when you don't know the full extent of a technique you can get beaten by the details.
edit: that said, i fully believe that kamui, esp complete, is superior to ftg by a significant amount. meaning kakashi imo would be able to kill minato as easily as he would be able to kill other characters, bar tobi ofc. however its pretty much canon that s/t techniques cancel each other out, so the skill with which the technique is wielded determines the winner and it's pretty clear that minato is more skilled than obito at that, at least at that stage. also, pretty much no-one can reverse kuchiyose bar minato, only jiraiya did it once by mistake.