First off, I’m not that familiar with the MINI turbo systems or a Jan tune but I do know a thing or two or five about building my own forced induction systems and tuning them myself. One of the first things I learned is to never trust a butt dyno. I cant tell you how many times I thought my tune was running perfect until I got on the dyno and was disappointed.
In response to your post I was specifically commenting on Garts situation. Obviously if you are running a heavily modified system with 20 PSI then yes, you would see a huge benefit with a larger IC for all the reasons you mentioned. Since Im not familiar with Garts car, I assumed that the IC was the only mod he had while running stock boost. Hence why I asked if he had seen a drop in boost, or turbo lag.
In my own personal situation I upgraded my intercooler around 6PSI and saw my boost pressure drop down to about 5.5. Max boost was also reached around 3.5K instead of they usual 3. It wasn’t until I started running 12+ PSI that I saw a increase in power. Now at over 22PSI I’ve maxed the efficiency of the largest intercooler that I can get to fit. Personally I prefered the instant boost and low heat soak of the smaller intercooler.
Remember, the air being pushed out of your turbo cant get through your throttle body until the intercooler is entirely filled with air. The larger the intercooler, the longer it takes for the air to get through.
Granted this is all under the assumption that the stock MINI intercooler is 100% efficient at stock boost. If its horribly undersized (which it may be), then you may see a very slight difference on a very hot day. But you would be surprised how efficient small intercoolers can be.