The cap uses a spring-loaded relief valve to hold cooling system pressure, and this is relatively insensitive to outside air pressure/altitude. I expect you'd need laboratory-grade instruments to see any difference. Going to a higher pressure cap may put undue stress on the hoses and rad or heater cores. If there is any doubt about the cap's behaviour, best to replace it with the stock 75kPa unit.

Strange temp gauge behaviour in post-85 240's can most often be traced to the little circuit Volvo incorporated to dampen out needle movement. There is a simple way to bypass this and get pre-85 gauge performance - check the BB archives, there's plenty of info and how-to pictures.

Could also be a failed-open thermostat. With the engine at full temperature (~10 minutes of driving) turn the heater temp on to Max and see if really hot air comes out. If not, probably a bad stat. Then check the temp of the upper rad hose. It should be too hot to do more than a brief touch.

In the absence of maintenance records that show otherwise, there are a number of items that should be serviced/replaced on a new purchase. Coolant and a new stat are among them. In your area, I would use a 92C stat - winter is coming! But check the owner's manual.
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Bob (son's 81-244GL B21F-M46, dtr's 94-940 B230FD, my 83-244DL B23F-M46, 89-745(LT1 V8), 98-S90, 77MGB and four old motorcycles)