In one respect, considering the amount of heat radiation area of the valley gasket, that little bit of heat deflection might not make much difference. What it really is allegedly for is to dampen harmonics in the valley pan gasket, so that it does not crack in the beaded area, causing additional noise and unusual engine oil leaks.
In another respect, leaving that area more open under the manifold can let heat dissipate better after the engine starts, too.
So, proceed with your DIY project.
Enjoy!
CBODY67