I tried 2 heat exchangers in series and saw the same negative results. I've run 3 different heat exchangers and they were all within 5 to 10º of one another in peak IAT's. The larger the heat exchanger the faster the temperatures fell. Ambient was within 5º of one another as well. Unfortunately my intercooler has a leak and I don't trust my data completely but it seems to jive with others experiences.

Les