How to determine if my 2015 MacBook Pro can still handle current workloads?

I would look at three things

1. Time is money
2. List specific measurable benchmarks ( e.g. launch 4 specific apps each day and time them till usable, script a clean build of a representative app )
3. List specific app and OS requirements ( e.g. need N-2 and N+1 beta compatibility for Xcode SDK)

In the end, you’re appealing to your manager for funding and reasonableness of the “requirements” and not IT for a “blessing” of the age or benchmarks.

Consider that people are expensive to recruit and train and hardware is cheap. But it’s easy to measure hardware expenses, so that tends to get scrutinized over process (erase and install) or training.

Do some quick math on “Is it worth it?” and good luck!

In your case we would send you an identical Mac to your 2017 one that is erased and have you run your chosen tests. Most of our managed Macs need an erase every 2 years and developers tend to need erases much more often since they alter the system so heavily. They understandably complain bitterly when asked to clean (just like system admin do), which is why we send them a second kit to run tests so they aren’t forced to migrate but can do a good test on a clean system. It’s rarely the hardware slowing them down in my experience, but when it is it’s nice to have good data on a clean build so that the budget person doesn’t think the “spec” was made up or not backed by data and testing.

Nobody except you cares for benchmarks. That was the wrong tree to bark up to.

What you need is specific use cases. "I need a new computer because task X must be done on the latest MacOS version, which is no supported on my current machine."

That it's faster is not an argument. That you can't do your job is.

So find which things actually REQUIRE a more recent machine. Ignore your own comfort or speed. Look for things that you actually can't do AT ALL without a hardware upgrade.

If you can't find any, then it's time to ask yourself if you really NEED a new machine, or just WANT one. Looking at the question from that perspective will help you find an answer that also convinces IT.

Depending on the corporate environment the case for new hardware may be difficult to make. Things which might work:

• Instead of using Geekbench and similar benchmarks (which don't easily translate to actual working experience) it might be better to argue with time wasted due to inadequate hardware etc. You might argue that booting up in the morning takes >5 minutes, or recompiling your applications takes 10 minutes or whatever, and that you use valuable company time just waiting for your computer to finish a task
• It might also be easier to convince your manager (who is more interested in your performance) than the system team (which probably has a limited hardware budget each year). The manager might have better leverage with the system team then
• Assuming you are not the only developer it might also help to join forces with your colleagues and make the case for new hardware together

With a company policy like that I doubt you'll gain any traction at all until your Mac can no longer run the latest macOS & therefore the latest Xcode.
If they're not interested in Geekbench scores I doubt you will find anything else along similar lines to compel them. "It works, so it is fine."

The only hope I could see would be if you could make a case for needing Apple Silicon.