Less velocity = less revolutions per minute (rpm). Formula: 12"/twist rate X fps X 60 sec/min = rpm A bullet traveling 3000 fps in a 1 - 12" twist is spinning at 180,000 rpm. A bullet traveling at 3000 fps in a 1 - 9" twist is spinning at 240,000 rpm. At 2500 fps in a 1 - 9" twist barrel, a bullet is spinning at 200,000 rpm. getting closer to the 1 - 12" rate barrel. So yes, less velocity = less spinning.
Another interesting note: when a bullet is spinning at a certain rate coming out of the barrel, it looses very little spin rate over the course of it bullet path. Even several hundred yards out. What does this mean? If it is stabilized coming out of the barrel it will stay stabilized through out its flight path, unless it is on the ragged edge of being stabilized in the first place. Just a word of warning; there are other factors that influence bullet stabilization, such as crown condition, how well the bullet is traveling down the bore (is it well centered and did not get distorted some where along it's internal path) are just a couple of things.
You might try to slow down a bullet, but personally I would load starting loads and work my way up watching for most accurate load. If you come to the end of the loading data (upper limit) and still have disappointing accuracy, I would switch bullets and try again. If you have a large cache of bullets that you must get to work, then using reduced powder loads would be a way to try too get them to shoot well. When reducing powder loads, you must be careful about which powder you are using. Some powders do not like to be under loaded - they tell you by having hang fires, wild pressure spikes, unexplained blown primers, poor accuracy, etc. If you are going to greatly reduce powder densities, use a powder that is known to be safe doing this. Good Luck and Good Shooting
