driving on the highway... you won't feel a large difference. Maybe a bit softer ride actually due to the extra sidewall flex.
Now under aggressive driving/cornering, the extra sidewall flex will cause undesired handling response, and excess roll in the tire (adjusted a bit with pressures)
Most people here are assuming your placing a much larger tire on the rim for performance purposes, if this is the case the large tire on a small rim is undesirable (again unless your drag racing as stated above).
Why do you think everyone uses a larger rim with a larger tire. I am currently running 235/40/17 on an 8" rim, and even that with R compounds is as large as I would like to go. And R compounds are typically known for having much stiffer sidewalls than street tires.
Typically, a 245/40 setup should be put on an 8.5-9" wheel Ideally. This range can be toyed with, however its not ideal.
In the end its your call, it's not commonly done and most tire shops will probably advise against it.