Diamond wire tension is another matter entirely. It is very likely that your mast is not straight because the diamond wire tension is uneven. Boat performance, particularly upwind, can be largely affected by diamond wire tension (i.e. mast prebend) on boats with swept spreaders.

I looked but could not quickly find an TheMightyHobie18 tuning guide reference on the net but once you have some basic specifications you should be able to measure your prebend a couple of ways. The BEST way is with a Loos gauge that measures tension on cables - adjust both diamond wires so they measure the same (somewhere, somebody should have a specification in pounds of tension). If you have raked spreaders, that is if they are made up from four bars that sweep toward the back of the boat, you can also measure prebend / diamond wire tension by supporting the mast horizontally on the ground with the spreaders vertical (one on the bottom pointing down and the other pointing straight up - this way gravity will not largely affect your prebend measurement). Run a light string from end to end of the mast from the tip of the mast luff track to the base of the mast luff track. Measure the displacement of this string at the spreaders. Adjust the diamond wire tension until the mast is even left to right and the displacment of the string, or prebend, meets the measurement.

The more serious racers will adjust their prebend to match the wind and water conditions for the day - more prebend (less power/more efficiency) for flat water and/or windy days, less prebend (more power/less efficiency) for light air and/or choppy water.