warm fuel has less power, some cars run fuel coolers
I think that holds true at WOT, you want to keep the entire intake charge as cold as possible to cram as much into the cylinder as possible (and colder = denser).
However, on an energy per mass basis, I think actually extract more energy from a given mass of hot fuel since you'll get a more complete combustion (hotter fuel = more volatile).
There may be tuning parameters that need to be changed - retard the timing (since combustion speed will be faster due to increased volatility), longer injector pulsewidth (less dense fuel = more time required for the same mass of fuel), but any modern engine should be able to make those minor tweaks automatically.
Also, Kyle raises an interesting point about the fuel boiling after engine shutdown, however I think many systems will retain pressure in the lines after shutdown. I know this happens on my car, but it could vary with different types of fuel pumps or leaky regulators or leaky injectors etc.
If your car has a manual transmission and you have replaced the radiator with a universal aftermarket model, chances are you have the perfect setup for a fuel heater- just redirect your fuel line to include the transmission oil cooler circuit in the radiator.
Just to be safe, you may want to air pressure test the trans fluid cooler lines to be sure they will take 40 psi.
i wouldn't use the oil cooler in a rad for fuel heating. although it seems a nice idea, you have to remember that your fuel lines are much smaller than the oil cooler in the rad so there is firstly that volume change to contend with, secondly you're adding a lot more restriction to the system by adding extra lines and running it though the cooler - can the stock pump handle that and still provide adequate fuel? i don't know myself but i don't think i want to find out.
I don't expect big gains form this as you'll see posed on many web site, but I do suspect there to be some gains. The possible gains would be from improvements in the injectors spray, like the gains from acetone, or HHO.
As to fuel boiling when the car is not running; my car holds some pressure in the fuel line when off. I found this to be true (all over my shirt) last time i changed my fuel filter, so that is unlikely to cause major problems.
The radiator possibility seems like it would work relatively well, but is a more complex approach. Also I'd rather not mess with my trans. temp. The radiator doesn't only cool, but it heats the trans. oil as it transfers the water temp to the oil.
I was thinking of something similar to the one in the posed vid (the first one). But with the copper tube just rapped around the rubber hose, and as much of the aluminum connector that comes out of the block as possible. Also the addition of insulation rapping the coil and everything post-coil seems like it would make the system more worth wile than that guy's.
The pump should be more than okay with the added length and the change in diameter won't be an issue once pressure is up and the fuel line is full but as was stated I would worry about fuel pressure. Nothing in the radiator is designed with high pressure(20+psi) use in mind.