The a/c core is the only thing that sits infront of the intercooler so when its out of there, you have 100% of oncoming air straight on to whatever is in the OEM intercooler position - thats the idea of the Forge Twintercooler - to catch all the cold air first.
I would think that an APR or THS with a/c delete would be as good an option efficiency wise as an S3 and forge, and it would work out cheaper too. Being honest, I would probably say that would be more efficient as its thicker than the S3 and will have no obstruction (twintercooler) in front of it.
Will need to look back at the logs, but they were done back to back on the same day on the same road with the same temp. 20 something degrees rings a bell.