Yes, according to construction and use regulations................
The vehicle shall be fitted with a speedometer capable of indicating speed in miles per hour at uniform intervals not exceeding the relevant miles per hour at all speeds up to the maximum speed of the vehicle and be capable of being read by the driver at all times of the day or night.
That's assuming that your 2610 is presumed to be, primarily, a GPS receiver and not a speedometer. The ironic thing is your 2610 is likely to be much more accurate than the OE speedometer.
Mounting it above the speedo, rather than obscuring it, keeps the unit much closer to your eye-line.
I've stopped using the voice prompt since fitting my NavII there - the flicker as it changes screens at the same time as it comes up with a voice prompt is enough to attract my attention and get the turn info...