I know of some very prominent pilots who turn their engine off deliberately in fights.
And, the windmilling phenomenoon happens now and then. Just an hour ago, I followed DblTrubl's 190D9 up in a rope a dope in my c202, since I had hit him, saw smoke and saw his props windmilling.
His engine wasn't dead - he had time to shoot me in the rope a dope before he lost all fluid in his radiatior and his engine seize.
Quite a good trick, turning off yer enemy to fool the enemy. It wasn't what DblTrubl did, but how I saw it.
So it does affect yer tactical thinking.
I think SKurj's suggestion is an excellent one: engine off would be instantaneous, but engine on should take a random number of seconds between 0-5 or so.
Tac: it seems when you're in flight, the engine powers up to max directly.
In some planes, there's a well known cheat: hit 'e' three times when you're on the runway waiting to take off, and the engine revs up instantly. I think it is related to the fact that engines seem to power up instantly in the air.
Only works on some planes though (know it does for 190's and 109's)