According to Hellmuth, the notion that he's bad at cash games is largely a myth - instigated partially by himself to get action.
Part 1
Part 2
He says he used to show Matusow a real donkey play, who would then blab to everyone about how bad Phil was playing.
Also says he is overall a big cash game winner (inlcuding the Big Game, though he only played 4 times last year). Certainly any earlier HSP failure or bad play (which people way over-emphasize IMO) is undone by his recent success there. (though I think him blaming playing a couple HSP hands badly on being tired is a bit shaky)
Now of course Hellmuth is a biased source...but then, I think people's judgements on Hellmuth as a great tourney/bad cash game player come from little actual data.
I think the notion that the best players outclass him and he just feeds on poor players but can't "legitimately" hang with big guns is a perception based on very scant actual information.