My Z is wheat intolerant so has the Arden Grange biccies, however he also reacts badly to colourings such as the deep red they use in a lot of the fishy chunks (Felix are starting this too). Whiskas is just too rich for both my boys period. Its usually back up within an hour
I'm currently contemplating the cost of switching Z to a raw diet. He adores raw meat especially chicken so it's a winner as far as he's concerned. RW doesn't like raw meat and neither like the supermeats...big chunks for big boys or they turn their noses up!
The boys are also prone to wolfing thier food too quickly so I tend to feed 4 meals a day rather than 2. If I switch Z to a raw diet that means he will have to work to get his meat off the bone so will eat more slowly. As for RW i'm considering Bozita or the one they sell in Lidl's (can't remember the name off the top of my head).
With most cats with intolerances its a case of testing them on new foods one at a time (and have a few cartons of a hypoallergenic food or chicken and rice to one side for when tummies get a bit sore)