I suspect they would thrive, The itchen has strong flows, clean gravels and is stuffed with natural food. However, in my personal opinion they should not be in there.
Lets be honest about this. The chalk streams are historically natural trout and grayling venues, not because they're only suitable for Trout and grayling, but because Man has conditioned them that way for nigh on 150 years. Pre that time there used to be far more coarse fish in the rivers. And why wouldn't there be. Fast, clean water, even temperacture, excessive amounts of natural food etc.
However, they are trout and grayling rivers now, man has made them that way and in the eyes of any angler of any age they always have been. They must be kept as such.
arble if found will be removed by the angling clubs quickly and efficiently, be it by electro fishing, or catch and kill. They'll be dealt with.
It sounds sad ut when these rivers can charge 450 a day for trout fishing then you can see why it has to be pristine. If I was paying that much I wouldn't expect anything less.