Use this Boyle's law calculator to estimate a gas's pressure and volume in an isothermal process.
www.omnicalculator.com
Say you have a 50G WH. Your inlet water temp is an average 50F, and you raise the temp to 140F. The water will expand about 0.75G. (see
https://www.watts.com/resources/planning/etp for this calculation ). Now, you may not dump the entire WH volume, so the total expansion may often be less than that maximum and the pressure wouldn't rise as much. LIve with teenagers that exhaust the WH tank, you'll see that, though.
With the Watts PLT-5 ET, the smallest they recommend and the boyles law calculator, if you started out with 60PSI, it should rise to 96PSI, but if you bump it up to the next larger Watts tank, the PLT-12, the same expansion occurs because the temperature rise is identical, the pressure would only rise to 72PSI. As I said, 115psi is too much. Valves and seals are designed for 80PSI max. Shower in the morning, leave for the day, the system could be sitting at the high point all day until someone uses some water to relieve it. You install a PRV to limit your water pressure. A properly sized ET lets you achieve that. (I rounded off a gallon to = four liters, which isn't accurate, but is a close approximation for demonstration purposes 1g=3.79 liters if you want a more accurate estimate)
So, yes, the pressure will rise while you're heating water with no flow, but it doesn't have to rise all that much if you size the ET appropriately. The science doesn't lie. Now, will your system survive? Yes, but hoses, seals, will fail more quickly and you'll be putting more stress on your WH. Keep in mind that the typical WH tank is glass lined thin steel. Constantly raising and lowering the pressure along with the heat (especially if it's a gas WH), will stress the lining even more. Keeping the pressure within a smaller range has advantages that, in the end will more than offset the cost of a slightly larger ET in both its life, and other things in your home.