although my undertray doesn't cover the oil pan and the exhaust it would mostely shield it from any direct airflow, because i didn't want to risk anything and because the exhaust would contact the undertray i made a cutout in the trailing edge of the tray to give them both some breathing space....haven't noticed any problems with overheating and such.
i might get away with better aero, but i wouldn't want to find out my mods cause trouble if i'm on holiday for example and my driving is a bit more demanding than on my dayly commute
o the other hand i've wondered if an aluminum undertray wouldn't actually function as an heatsink and improve cooling. if you do a full undertray you might also consider louvers below the parts you want to keep cool they will actually extract hot air at speed