Not really, but not quite either!!
There was a thread Farrell made which explained it better than I can!
Here goes anyway!
Basically it's important not to confuse grip with traction and be aware that neither imply better handling!
Read the above line again!
A wider rear tyre will have a wider amount of rubber touching the ground, but the ratio between the width of the patch (235/255/265 etc) and its length (the 2-4cm ish front to back of the contact patch) will change too, which affects the overall grip of the tyre.
This means that although traction should be improved in acceleration and also one would hope in cornering, it would also make sense that if (say in a constant radius corner), the lateral forces the tyre can cope with would be more in a wider tyre. But, as the speed (or more correctly; load) would be more before the tyre reached its maximum grip and started to slide, there would be more lateral load being placed on it when the tyre slides, resulting in a faster and heavier sideways slide. To cut that very short, when it lets go, it lets go faster giving less time for the driver to respond.
Before everyone hit's "reply", bear in mind that a wider tyre also means that the weight of the tyre is spread over more area, so less pressure is locking the tyre to the ground.
It is possible to "over-tyre" a car, which many boy-racers have found in the past, their cars performing worse in corners than stock with wide-boy aftermarket wheels.
Also, different compounds, makes and models of tyres will have different characteristics, so will vary in the way they work, some gradually giving up the ghost, some squealing like made before a slide, some deathly quiet and some grip like mad then bang and nothing!
Now factor in the wider tyre being heavier than stock so the unsprung weight of the wheel assembly changes, then the sidewalls being under slightly different stress levels given no change in the J width of the rim, you can see that it's easy to mess around with overall grip and traction but getting a cars *handling* to be improved is a delicate dance indeed.
Without citing any actual references, I seem to recall some members having enormous fun on trackdays with their cars on factory 235 all-round tyres as the 3.6 car's suspension and antiroll-bars were originally designed for this size and it balances their cars very well.
Many owners, myself included, found the optional staggered 17" combo of 235/255 very prone to understeer. BMW agreed and the 3.8 cars with factory staggered rims got a increased rear roll-bar diameter.
I'll shut up now, especially as I don't run the proper size wheels on my car anyway, and this thread is supposed to be about the choice of track wheels above and not go off-topic into another handling thread.
Best wishes and I hope my ramblings made some sense.