Upgraded to 5.8 on Friday. I really like the improvements to the Maps/Nav - especially the option to go 'direction up' instead of always 'North up' on the map orientation. Performance on the map refresh and map traffic details seems better as well. Regarding the lowering, possibly in combination with the winter tires on, I seem to be getting more wind noise from the wheels or turbulence under the car at speed that I had not noticed before.
Tried tethering while parked in the garage - definite speed improvement over 3G as cellular reception is only 3 bars at my house.