regarding GPSMAP 66ST (singleband) and 66SR (dualband), here is a comparison at a reference point as a static survey,
66ST is showing 4m as oem GPS accuracy, the real (error) accuracy is 3.004meter
66SR is showing 1.8m as oem GPS accuracy, the real (error) accuracy is 2.389meter
the real position is this one: N 49.43093010° E 10.94170046° (~1mm accuracy )
it is very far from a professional device or even from a MI8's accuracy (i could reach subdecimeter accuracy (both H/V) with MI8 with postprocessing), the MI9's GPS part (the 855 inbuilt one) is a huge stepback comparing to MI8's additional Broadcom chip, probably You will have a slightly better track when You are moving but wont be accurate (in my eyes).
BTW, does somebody know which dualband chip is builted in the 66SR? the new Sony dualband chip or any other , i didnt find info about it (if it is the new dual Sony, then it doesnt worth a penny :))