I fixed the vacuum, tapping into the hose to the headlight lifters.
There is a noticable difference on the engine in practical driving with the hot air device fitted. Cold starting is easier and it runs a bit better cold. This is probably due to the reduced amount of condensation of fuel in the manifold as Roy mentioned. Warm up time is also improved - the engine is now noticable quicker to warmup. Also predicted by Roy. It wasn't bad before, but now it is even quicker. City driving in low rev's and idling is also improved with a more stable and slightly better sounding idle.
Hot air burns faster than cold, so by allowing the engine to breath warm air, a similar effect as advancing the ignition may be accomplished. As I do a lot of city, low rev, low throttle driving, I expect a slightly improved fuel economy because of this.
Some might think the hot air will reduce performance. True, cold air contains more oxygen per volume than hot, but first of all, the temperature isn't that much higher (e.g. a temperature increase of 20 degrees results in only about 7% more molecules per volume) and besides, in full load situations, the device switches over to cold air anyway. This is because at full throttle, vacuum in the manifold is reduced and even if was still breathing from the hot air inlet, the increased airspeed around the exhaust will mean less temperature increase.
Winter will tell the difference regarding carburettor ice, but so far, I can say that if you have the device, don't consider it insignificant! I have learnt something interesting by running without and now with the device