I don't see much problem in having a stress test if you are concerned. BUT, be aware that there are false positives that can really mess up your life if the cardiologist isn't first rate or if he has his pocketbook as his first concern. Had I not been knowledgable and assertive I would have been put on a life altering regimen and drugs after one of those false positives. But, working through the situation was time consuming and expensive.

If you are healthy and are, as your post says, only concerned with maximum heart rate, nothing else I'd approach the need for further diagnostics very carefully. Understand your provider may not be able to give you any better answer and may subject you to expensive testing for liability protection reasons only.

Maximum heart rate can change over time. Mine has increased 20 bpm in the last three years as my fitness has increased. When I started running years ago I was taught to do most of my exercise in the Aerobic Zone. That is I could always comfortably talk while exercising. But there were periods of Anerobic Activity where I literally was gasping for breath to develop greater capacity. The combination worked for top flite runners then and still is a valuable tool.

In short, don't pay much attention to maximum heart rate except ast an aid. The best thing is to pay attention to breathing.

Understand, this advise may, just like most internet advise, be worth exactly what it cost.