Hi Robin,
I would leave well alone if you have a rolling road booked you will just be guessing even if you use minty lambs excellent charts. If you ring Burlins technical department they will advise on everything except needle selection, it is only on a rolling road that final selection can be made. I would expect Hs6s to be less economic the CD150 as they have a 1/4 inch larger choke, indeed they are fitted to increase fuel flow hence lower MPG. So if the car will get to the road leave it alone!
Mick