I agree it shouldn't be that in order to achieve maximum speeds you have to purchase more equipment. The SH3 is, specification-wise, more than capable at delivering a fast wireless connection with enough headroom to serve a good few devices with maximum speed before reaching any limitations as my SH3 has proven over the last 2 days. I am still getting 100Mbps+ on all my wireless devices even if I go downstairs where the signal is not at its best and perform tests on all my devices at the same time.
What I may do in order to test how good, or bad, my 3rd party wireless router is take out the SH3 factor and test my 3rd party router at my dad's house on his SH2 to see if it can push out 100Mbps+ there. It would only prove that my 3rd party router is capable, or not, but would be a very valid test nonetheless to see if it is just my 3rd party router acting up with a hardware/software issue. It would at least rule out one reason.
We could go on for weeks both throwing back and forth completely logical and valid reasons as to how it should work with a 3rd party router, but as you mentioned, my SH3 has shown that it does not follow the laws of logic and common sense and appears to be forging its own random path.