Some info from my observations.
GPS accuracy compensation table. Where this data is stored? I think this data can be stored only in NVRAM (region 41). I have Nuvi 3790 flashed to 3490, and I never have had accuracy problems. For some time my Nuvi was with original (3790) region 41 but after some time I flashed region 41 from 3490. Still no problems with accuracy, despite the compensation data changed (I think so).