I get about a 20'F rise in charge temp under max acceleration in 3rd gear. Colder ambient temps see less of a rise and warmer see more rise, but the average is around 20'F with the tune and stock intercooler. It is less than that with the Mountune intercooler and the time it takes to return to ambient is much faster than the stock intercooler. If I remember correctly, timing starts to get pulled when the charge temps are above 110'F. For me, that would take an ambient temp of 85-90'F, which is very rare. In 3rd gear, the car is already exceeding the posted speed limit and the intercooler can start the return to lower temps in 4th gear as the engine is no longer producing enough exhaust gasses to create significant boost.
I haven't been on a road course with my ST to see what the Accessport reports for charge temps. With the Fiesta Movement ST's, we had been running a ST pretty hard for filming a commercial in Brazil where the ambient temp was very warm and the car spend a great deal of time idling. There wasn't any noticeable loss of power. We ran another ST very hard at the Nashville Raceway in-field road course, along with repeated drag runs and slaloms on a very hot day in June without any issue. The same ST ran 35 consecutive autocross runs and the tires gave up before any loss of power was noticed. The same car again made a few trips to GingerMan raceway for filming without any issues. Those were stock ST's and I didn't have a method to monitor charge temps. We had more issues with overheating the tires. I wish we could have monitored the charge temps to really see how a stock ST does compared to a tuned one. I knew back in 2014 that the weak link to making consistent power was the factory intercooler, but we never had issues. Others running ST's in California and Florida also didn't report any problems, and many of those were used for filming at Sebring and Horse Thief Mile/Willow Springs.
My advice is to monitor charge temps, ambient, and intake temps on your car and your unique use case. Gather the data for how your car is used and how you are using it. Upgrading parts because a dyno says they are needed is only valid if you drive your car on a dyno. Collect data that is unique to your application. Working as an engineer for a very long time has proved to me that unique application cases and data collection are critical to making decisions. I was having beers with a Ford engineer a few years ago talking about use data and how people surveyed would tell you how they are using their cars, but the actual data from their cars proved something totally different. Collect your data and make your decision based on that data. You may need to get an intercooler right away or find the stock one is ok for now, but needed in the future if your plan to further modifications, or the stock intercooler is just what you need.