As a GasSaver, we tend to watch our gauges more closely. You do run the risk of overheating in a normal way like any other car. You will read that people modify their grill block based on the season. Larger openings in the summer and such.
My car likes to overheat, so I installed a manual fan radiator switch. Since my fan is electric, I can just turn it on if I think the engine coolant temperature is getting too high. The ECU/PCM already does this on my car, but only when it reaches a higher temperature than I am comfortable with. I only need to turn the switch when I am caught in traffic jams and at stop lights.
My (owned, but not yet driven) car is a 1998 Jetta TDi (turbo diesel). It is known to run hot, and the owner's manual goes so far as to suggest letting the car idle for two minutes after spirited driving. I will never partake in this kind of aggressive driving though, so I am not too worried about letting the car cool down. This WOULD change though, if I were to make a grill block for the car.
Thus, I will probably opt to NOT make a grill block for my car. While the gains may be substantial, it is not worth risking permanently damagine either my turbo and/or engine in the process. If I can figure out a way around this, perhaps I might consider a block. Otherwise, I plan on finding other ways to improve my FE on the TDi.
On my 95 Metro only half of the opening actually allows air to the radiator. The other half of the opening has a metal wall about a foot behind it so it serves no purpose other than a symetrical bumper. Blocking half of the grill would improve effeciency while doing nothing to heating issues.
Inspect your opening like such to determine what is tolerable..
I have one of the guages on my Scanguage for engine water temp. only time i notice it gets up there is stop and go traffic. and the few time it did get up there i found that my electric fan kicks in at 220 degree's temp. besides that as long as car is moving i am ok.