Just my experience, looking at data over the past year, all at Summit Point WV.

AiM MXm, dash mounted (minimal view of sky), GPS accuracy ~5.5'.
AiM GPS08, roof mounted, GPS accuracy ~1.7'
AiM GPS09, dash mounted (good view of sky forward half), GPS accuracy ~1.1' (Never under 1')
AiM GPS09, roof mounted, GPS accuracy ~1.1' (Some short amounts of time at 0.9')

 

I think that will match what many people see for accuracy. Depending on the day and conditions, I see GPS09 sub 1' on a regular basis.