yyz340c re: eye / GS aerial height. I recall that there are design recommendations which indicate acceptable values for aircraft installations, thus limiting the extent of this error – possible the value was 5 ft in the approach configuration and at the approach speed.
PAPI has a relatively narrow beam as shown in the Canadian diagram; if you wish you can calculate the height difference between the WW/RR changeover points (angular difference and slant range), thus bound the aircraft altitude where the PAPI and GS should (could) agree. However, the most significant difference is that PAPI quickly ‘switches’ between W/R due to its design characteristic (sharp transition), whereas VASI suffers from a large ambiguous zone of ‘pink’; thus the advice to be cautious of VASI.
It is unfortunate that the Canadian section is titled visual approach slope indicator 'VASI' where they are discussing PAPI (APAPI), which is to be their standard.