DJ
I am not really sure what you are saying. In a sim if you did that turn (in a steady state wind) there would be no drop in IAS - I don't know what you mean by an instantaneous turn - they simply don't happen (you would need infinite energy to do that). The turn would proceed as per normal (in fact i have done this in the sim).
Capt Stable.
What you say in regards gusts is true, but it has nothing to do with turning downwind. The same can (and does) happen turning upwind, crosswind or not turning at all.
Gusts and IAS fluctuations is well known.
In regards turning downwind, even in a moderately stable wind there usually will be no change in IAS. The only change in IAS is due to a gust NOT the fact you are turning downwind.