Without going into a lengthy treatise on stability, the short answer is that a 'rolly' boat as you describe tends to have lower initial stability and higher ultimate stability. A 'stiff' boat tends to have higher initial stability and lower ultimate stability. In a sailboat you usually want high ultimate stability for carrying sail and tolerate the lower initial stability to achieve this. Therefore your Tahiti Ketch
is probably set right for ultimate stability and you should probably not be trying to change the stability characteristics without consulting a naval architect or at the least doing a roll period test.
Second, the ketch was designed with a specific sail area for the main and mizzen to set the 'center of effort' of the sails
in the proper releation to the 'center of lateral resistance'. Removing the mizzen will cause the 'center of effort' to move forward and impart undesireable sailing characteristics such as much more weather helm
. Increasing the length of the boom to add more sail are aft will help but probably not return the 'center of effort' to it's original location.