Sadly the range of Velvia is far less than 5 stops. Shadows record detail 2 stops below mid-tone. Safer to count on a range of 1 1/2 stops above mid-tone for highlight too for Velvia 50. 1 2/3 above mid-tone is pretty white! I reckon Astia has about 5 stops, but nothing like the saturated colour palette.
This is why ND grads are an essential tool for the transparency brigade.I use colour neg if the range is too much and cannot be controlled by ND grads.
Be careful shooting dawn and dusk with the Velvia 100, it will give very red skies.
Not up in London that often, but do pop into Teamwork when convenient. Some of the sales advice I have heard there has been 'creative' to say the least!