The only time i have noticed heat soak is at standstill, once you are moving and there is air flowing through the intercooler, the temps drop very quickly below 40 degrees, Unfortunatly there isnt a great deal you can do with the front of a S1, especialy if you dont want to cut about the front of the car. The thicker rad does work as efficient as the standard height item, how ever once you are at a standstill again the heat build up does happen quicker due to surface area being smaller, so any breeze that 'may' help keep it cooler in traffic isnt as helpfull.
I have fitted to mine a 9" kenlowe fan, fitted on the front left of the rad and a Pacet 10" fan on the right hand rear, when both are on there is ALOT of air being forced through the rad, Temps do drop fairly quickly, and normally in traffic the temp guage, with both fans on, on a hot day wont go over 3/4.

I have designed mine in this way as its predominatly a track car and is hardly ever used on the road, but there is no reason why it cannot be fitted to a road car especially if its only running a stage 1 chip. And its something abit different. Its the first time i have seen anyone trying to progress the intercooler options for the S1 in along time.