it really depends, for example 100bhp in a small car like a polo is plenty, but drop that in a big saloon or 4x4 and you'll find it severely lacking.
i've always tended to preferr a larger engine to car ratio, in my experience a tiny engine in a big car can be just as bad for mpg (because you really do have to drive wot everywhere just to get the damn thing to move) and with the smaller engine you don't even get the power benefit for your lack of fuel effeciency.
i do think there's a problem though with this fashion for cars with small engines and big turbos to get the power and effeciency figures, because whilst the numbers look good on paper it becomes a very digital experience, it's either sucking fuel and motoring on, or it's sipping fuel and barely moving. this sounds fine if you're in a lab or you live in holland, but in the real world with traffic and hills you'll be stuck somewhere in between the 2 extremes.