Joined: Feb 2007
From: Pretoria, South Africa
I Ride: '07 ZX10R Green,'07 ZX6R Orange
It does affect the handling for sure, if your front height is suddenly reduced by 6mm, as explained by KawiT, it affects the Rake and Trail of the front suspension - the same as reducing your preload on the forks, also it depends a lot on the radius profile of the tread surface - usually the different aspect ratio will have a different profile - affecting the handling.
The dealers accidentally put a 50 profile instead of a 55 on the rear of our ZX10 when we replaced the rear tyre, and there was a definitive difference in handling, turning into a corner slower.
A lower profile would also mean that the tyre should be more stable on the rim therby you should be able to run slightly lower tyre pressures, but as you can see, and as mentioned above, if you are not racing the bike, and if you are not intending spending a lot of time and energy to experiment, stick to the stock size tyres.
"Too much power is just enough!"