Aerobat 77
I think you are confusing yourself here. The limitations on CAT1 are in RVR, not met vis; if the particular runway minima allow it you can make the approach in an RVR of 550m PROVIDED the approach is flown in auto coupled mode, or manually with a Flight Director, or with a HUD. If you don't have an auto couple capability or a FD or a HUD then you are limited to 800m RVR.
NOTE RVR - not met vis. The met vis can be considerably lower than the RVR figure, as you say, and it's why RVR was brought in in the first place; it is RUNWAY visual range NOT met vis as observed from the met office.
Originally RVR was obtained by a man in a hut alongside the runway touchdown point and he literally counted lights! Nowadays, of course, it is done by sophisticated light measuring equipment usually at 3 places along the runway.
Luc Lion gives the reference authority to convert met vis to RVR which is called Converted Met Visibility. This is only used if RVR is not available or is above (I believe) 2000m
Hope this clears it up