The temp rise will change your optimum altitude. If the temp rise puts you above your optimum altitude for your gross weight, fuel flow will increase signicantly.
Elroy, not sure where the following info fits into the argument, however:
On the 737NG for instance, a change of OAT on the Perf Init page of the FMS (The FMS doesn't auto update this...), will lead to a recalculation of the Max Alt, yet not the Opt Alt.
As well, there are cruise performance charts (containing both max and opt altitudes) for ISA and below, ISA + 10, and ISA + 15. These charts, especially at heavier weights and higher temps, show a progressive change in the Max Altitudes, but the Opt Altitudes remaining the same. (Indeed, on the 26K-engined 800NG, at high weight and ISA + 15 degrees, the Max (1.3G) Altitude in some cases is LOWER than the Optimum Altitude. Weird.
Not sure why temperature doesn't affect the Opt Altitude calculation. It would seem intuitive that the increased climb duration in higher temps would lead initially to a lower cruise, or more appropriately, Optimum Altitude.