My 106 was very stable at high speeds, drove straight and true, no vibrations and could take your hands off the wheel if you were mad enough!
My speedo was only couple of mph out at 110mph on the rolling road and also ties in with speeds that other cars do, so I am confident that it was pretty accurate.
However, I know a 106 GTi that was reading 140mph at the same revs as mine in 5th gear, hence his speedo was out (by about 10mph) and I can see this being the case with this 106 were discussing here.