Absolutely order the backstay adjuster. They are comparatively inexpensive when you think of the cost of the boat. Backstay adjustment is more important on a cruising boat than a race boat. On a race boat you have crew to run around and re-tension halyards and sheets in a gust to balance the helm and reduce heeling. A backstay adjuster allows for a precision adjustment without leaving the helm. You can simply adjust until the helm goes neutral. Obviously it will be less effective in a slightly outdated design like the Tartan 3700 than on a more modern design but it will greatly extend the wind range of the sails without having to reef. It also allows a setup of the
rig with less backstay tension except in heavy going which reduces the strains on the boat and improves light air performance.
Jeff