If the motor is made to pull to redline, then pulling to redline is not "Severe Duty". But pulls at the Drag Strip, or the equivalent on deserted roads, can be if frequent enough. I would put 30 such pulls on the cusp ... it's far from easy on the motor but if the mill is in good state of tune and adjustment it's not a huge issue.
Now, if this was a regular "Soccer Mom" vehicle I agree, that would be Severe Service. But on the Mustang, these are built to handle the level of WOT runs you are putting on the car. Now, that is my opinion, but it's based on my experience. My Miata sees full throttle pulls to redline just about every time I drive it ... in other words hundreds of times between oil changes. It has over 200,000 miles on the original motor and there are no signs of it dying anytime soon.
My company had a television show filmed and part of the deal was we had a new Ford ¾ ton featured in it. I was able to speak to the Ford people about the vehicles they send out for testing to journalists (mostly) although our truck came in under the same program (it was a 2014 model but not precisely the same as might be offered for sale; this was spring 2013 and the spec wasn't final at that time). We tried to buy it off them but these vehicles are strictly not for sale, apparently.
In any case, I was told what Ford does is rotate in a replacement vehicle at around 2,000 miles. It's the same with all the vehicles ... they are thoroughly beat on and have a LOT of Wide Open Throttle pulls, but the Mustangs are particularly beat on. I was told that on the Mustangs they have to replace the rear tires more than once, and usually the fronts as well, before those 2,000 miles are up.
So these cars are run with the factory fill oil and have probably hundreds of pulls like yours has in those 2,000 miles. Not every automotive journalist can shift a manual transmission properly or has driving skills, so it's not just hard pulls it's ham-fisted hard pulls in many cases. The cars spend a lot of time right up against the rev limiter. And so on.
And they run fine, and Ford doesn't change the oil. My guess is the other OEMs do it very similarly.
It's not a big deal in my opinion that you have "lost track" of the mileage. You can instead go by age (or engine hours, etc). With full synthetic I would change once per year or once per x miles as appropriate, whichever comes first. If it's conventional (non-synthetic) I would be doing bi-annual (spring and fall) changes.
I have vehicles that won't see enough miles in a year ... it's one of the side effects of having more than one vehicle ... to justify a change based on Oil Change Interval (OCI) miles. So I always change based on how long in calendar months the oil has been in service, and I change the filter once a year.
Ideally (and others have disagreed with me on this) I like to change before storage or before winter if I'm doing an annual regimen. So you could change now, fill with synthetic, and use that as your basis for further changes based on elapsed time on the oil.
You should seriously consider beginning to utilize oil analysis. You are at the perfect time to start. That will answer a lot of questions such as yours, plus probably save you money since with any high performance vehicle, something can always break, even with relatively low mileage, and the analysis will alert you to some potential issues before they become expensive issues to remedy.
