I spoke directly with MT about their SR line. They said the feedback they are getting on mileage is ranging from 18k to 24k. It also is a new carcass that was designed for 17" and 18" rim sizes that they down sized to meet 15" demand. They also said they do not use the UTQG rating guide for the SR line because they are a niche tire that is not a drag tire nor cruising tire, but uses softer compounds than the ST line of pure street tires. Not that it matters much since the UTQG is only relevant to a specific manufacturers product line and is not a universal qualifier, despite what many sanctioning bodies require, and since a majority of MT products are race tires, their whole window of UTQG ratings could be biased compared to others manufacturers.

Now, something I have learned since that conversation that I have not gone back to ask about was if MT did a Step Up Speed test at all or if they did an SAE standard test to failure. Doing either of these can provide additional information about where the threshold of speed related failures actually lie since a speed rating is done at rated load, at the target speed, for 10 minutes, then stopped and the tire inspected.

Also worth noting, since you mention finding out how fast your car might be, the ECTA allows H rated tires to be used at speeds up to 175 mph on their 1 mile courses. They do not allow dedicated drag racing tires. V, W, Y, and Z ratings are allowed up to 225 mph. After that you are into nascar or LSR spec tires. So to extrapolate that into a one mile run to 150, you are spending about 20 seconds at top speed before you have to start the shut down procedure. That is obviously much less time than the speed rating is tested at, hence the deviation that ECTA allows.

If you are going to do autocross, track days, or even high speed events and are sticking with 15" tires, it isn't a stretch to have an extra set of wheels/tires specifically for that. Aero, Bassett, Diamond and others make relatively inexpensive competition wheels in a variety of offsets, finishes, and colors. Combine that with correct lug nuts, an A6 or R6 Hoosier or a Goodyear oval track tire if you want to hit some top speed events, and you can have mix and match sets capable of a variety of tasks that probably exceed the capability of your vehicle, your nerves, and can be changed out in 15 minutes at the track.