Hi,
A signal between a couple of planes is always routed as STRIPLINE, in your case
L2------------------------- (GND) <-- ground plane
200um core
L3------------------------- (signals) <-- Stripline signal
800um prepreg
L4------------------------- (VDD) <-- power plane
MICROSTRIP is when you have only one reference plane, like upper trace (L1) or lower trace (L6) on your PCB,
When you say high speed signals, what do you really mean? speed rate? MTs? are they differential or single ended? control or data signals?
Also remember that traces at high speeds behave like transmission lines, with all their effects (mismatch terminations, rise time degradation, Crosstalk, ISI, ringing, overshoots/undershoots, operation modes, etc).
As I know, memory is always differential, if this is your case, cheers to you, differential has many signal integrity advantages than single ended. but you need to understand trade offs and routing restrictions and follow some rules of thumb when designing your PCB. (it's good to see that for high speeds like you said, you have kept ground and power planes directly adjacent).
Eric Bogatin, Howard Johnson and Stephen Hall SI books are very good references on this topics.
On your final question about your tool (honestly, I don't know how it works) but, one way to calculate Z0 is using formulas for symmetric or offset striplines (handy formulas comes on Hall and Johnson books) they use widths, heights, permittivity and thickness for your calculations.
A second way of calculating differential or common impedance is using a 2D field solver.
- jc