Temps shoot up too what number? I wonder if you have something else going on. The t-stat by pass should only be in effect until the engine warms up anyway. I can tell you that with my heater valve closed and watching on a scan tool that my temps don't fluctuate much. I do see 215 sometimes until the fans kick on ,but that's plenty normal. Didn't want you to overthink the heater valve thing too much though. They're are plenty of off road guys running the heater hose ports plugged off with no problems, myself included. I am curious what your temps are with the heater valve closed. The heater core ,as you know, does act somewhat as a small radiator. It would not surprise me if it dropped you temps some, but I would only expect a reduction in temps if you were running the heater fan on high and pulling heat out of the core.
That pump could easily produce that kind of pressure if there were not flow as in the t-stat providing a restriction to flow. I work as a Fire fighter and our trucks all have centrifugal pumps. They will make over 50 psi of pressure in the system when engaged at idle with no water flowing, so to see it on a much smaller scale with a water pump is totally possible. You could always put a pressure gauge on the cooling system to verify?