I've noticed the same performance. I am in South Louisiana and work shiftwork so I drive in the coolest and hottest parts of the day. I've wondered if just upgrading the intercooler would help (no tune) but increasing intercooler volume would mean the turbos have to work harder, spin faster, to build the same boost over the larger volume (from what I hear, our turbos are about maxed out on performance from the factory). So I wonder if having more intercooler surface area is preferable to more intercooler volume (thicker intercooler vs taller intercooler). I do plan on doing a Whipple kit in the next few months.
Regarding intercooler volume, I'd consider it to be relatively insignificant in light of the difference in volume between the factory IC and an aftermarket IC. Let's say for the sake of argument that the turbos flow 600 CFM at full boost (I have no idea if this is accurate, I'm just using it to make the math easy)... that's 10 cubic feet
per second, right? Let's now say that the internal volume of the factory IC is 1 cubic foot (again, I have no idea if this is accurate, I'm just shooting for easy math)... so it would take 1/10th of a second to fully pressurize the factory IC. Now let's increase the volume of the IC (via replacing it with a larger aftermarket unit) by 50%... so now instead of completely pressurizing in 1/10th of a second, it takes 1.5/10ths (or 15/100ths) of a second... a whopping 50 millisecond difference (literally 17% of the blink of an eye).
Of course, something else worth noting is that it's not like the IC has to be brought fully up to pressure before the engine starts seeing boost... it's not like a pressure regulator where it will only open up once the target pressure has been reached... it WILL pass along pressurized air at some ratio to what is being fed into it (you obviously lose
some pressure across the IC), but in the grand scheme of things, I think going to a bigger IC is comparable to peeing in the ocean (in regard to boost response)... sure, there's a theoretical difference... perhaps even one that could be measured by precise instrumentation... but in regard to the user experience, I would expect it to be virtually transparent*.
*of course, if we get silly with it and use, for example, the volume of a warehouse as the volume of our IC, there would obviously be a HUGE impact to boost response... but realistically, that's not what we're talking about here
