A 5" core isn't going to provide you with maximum IAT reduction, a 7" core will
We're releasing a 5" core in the next couple of months and I do believe a 5" core is exactly what some people need, but I also believe you shouldn't have to buy a product twice. There are plenty of people that will purchase a 5" core and then eventually upgrade to a 7" or a larger core simply because their goals will change. I've seen it on many platforms, especially young platforms such as the N54. We're just getting into turbo upgrades & big singles and in the next year or so, there will be quite a few more options available that will require better cooling.
The 7" is ideal for anyone running an upgraded turbo, such as RB upgraded twins or a big single. What I find is most people make the mistake of being too short sighted with their mods. While a 5" core is going to do a great job on the stock twins in the low-mid range, a 7" core is going to do a better job, especially when pushing the twins out of their efficiency range at higher boost pressures.
You do sacrifice a bit of pressure drop, but running a lower IAT is always going to be more beneficial in comparison to losing a few .x PSI. Running cooler temps allows for more timing advance and produces more power. Lower IAT's are also more knock resistant which results in a safer/more reliable motor. Running an smaller 5" might result in a little less pressure drop, but you're sacrificing cooling, power and knock resistance.
Most of the 5" intercoolers on the market are more than enough for the average consumer who's only looking for a few bolt ons to add power until their lease runs out, but for the guys looking to maximize their setups and make the most possible power, the 7" is the best way to go.