You should not play around with rake and diamond tension, until you have a good understanding of sail trim. E.g. When I bought my boat I sailed around 50 times until I started to understood how it reacts on downhaul, sheet, rotation and batten tension. Then I could understand how the boat reacts on different rake and diamond tension setting.
IMO the best reading is on the landenberger page:
* More rake = more stiffness in the longitudinal axis and more pre bend
* More diamond tension = more pre bend
And as Matt said, rake and tension work in the lower part of the mast/sail.

Once you understand why you are slow (sail to full or flat and where; you need tell tales to see this), you can change rake and diamond tension to adjust.

If you are not sure about all the stuff above, make some pictures of your sail in strong wind and light wind trim upwind and in downwind trim (boat on the side, mast supported at the hounds) and post it here.

Cheers,

Klaus